Ew. Binary effect resistance is a worrisome thing with even half-decent spells (charm/hold monster). 3e had descriptors so things could be immune to Mind-Affects, everything powerful was expected to have a save (and Irresistable spell was 3rd party), and SR was expected to be a pain to overcome. SR, of course, wasn't enough but its easily fixed in my sig since the underlying mechanic is fairly sound (AoE's are the only part that may or may not have been an oversight).
The monsters themselves have condition immunities instead of the way 3.5 does it where the spells have descriptors and certain monster types have immunities to said descriptors. Appendix A of the 5e PHB covers the condition definitions, and the majority of control spells have these conditions coded into them. Additionally, most save-or-sucks allow repeated saves to end the effects early, so they're not nearly so binary.
Particularly magic-resistant monsters have saving throw proficiencies in addition to their Magic Resistance trait and have lower HP to compensate. Martials come in handy for those monsters. A balor is not outright immune to save-or-suck effects, but the odds it will fail a saving throw against a spell that could disable it with a Str +14, Con+12, Wis +9, Cha +12 saving throw line with advantage on saving throws against magic is slim against an expected spell save DC of 19 by the level one would expect to fight a balor. If that's not enough, there's the legendary resistance mechanic where an exceptional creature can choose to pass a saving throw it would fail X times per day. The end result is all this stuff works on the same mechanic, the saving throw roll, and it's just a matter of probability doing its thing. On the player's end, it's only one defense to get through, even though several mechanics are in play on the DM's side.
Devoted Tracker: The paladin's special mount feature is disguised as the spell Find Steed. A paladin's Aura of Courage (+Cha to saves for the paladin and allies within 10 ft.) covers the mount pretty well and the feat Mounted Combatant leaves only Constitution save aoes as a significant threat to a mount. Depending on the oath archetype, the paladin may have additional auras to protect his mount, such as immunity to fear for Devotion and resistance to damage from spells for Ancients.
Swift Ambusher: There's no skirmish in 5e, and nothing has immunity to Sneak Attack or equivalent abilities. Undead, plants, constructs, etc. can be the victims of Sneak Attack with no special setup compared to other monster types. Creature types do not come with any inherent immunities or resistances, and instead such things are written into the damage resistance, damage immunity, and condition immunity lines of monsters on a case-by-case basis. Resistances and immunities are generally confined to the damage types listed in the PHB (with caveats for weapons) and the conditions listed in Appendix A.
Sure. I tried but found it difficult to dive into 5e for many reasons, many the porting or lack thereof. I don't mind continuing over PMs/skype etc. I suppose some basics would be a spell conversion template (they are still the main source of power, right?), some gameplay concept conversions, a monster conversion template, a class conversion template, feats would have to be done individually, a prereq conversion template, item conversions might be templatable...
Top tier classes are still spellcasters, but spell list limitations define and balance the casters in a much more strict way in 5e. Wizards have a better spell list than every other caster, and without that spell list, their features would not bridge the gap. Similarly, the bard is higher on the tier list than its spell list would suggest because Magical Secrets lets it steal from every other class. Druids fall off considerably past the most-typically played low levels because they have the weakest spell list of the full-casters and wildshape scales poorly, such that by level 10, the half-caster paladin is more desirable than the full-caster druid. Eldritch Knight, the gish archetype of the fighter is actually lower tier than the warblade-equivalent Battle Master because the maneuvers are generally better than the level of spells available to the Eldritch Knight archetype, and Eldritch Knight is still considered good even though the Battle Master is better.
My own experience says don't worry about porting until you've actually played 5e awhile so you have a feel for what 5e is trying to do and what it offers. Classes are packed with features, especially at low levels, and you get a lot of customization from multiclassing as opposed to feats. Trying to force it to be 3.5+ is an unsatisfying experience and you're better off continuing to play 3.5 if you want to customize via feats and prestige classes. After 2 years of DMing and seeing the combinations possible via multiclassing, I don't think prestige classes are necessary at all for 5e. Archetypes fill in a lot of the mechanics prestige classes used to, so I'd be more inclined to port a prestige class as an archetype of a base class if I could not achieve the same mechanical feel via multiclassing.
You don't want to do a straight conversion of a 3.5 monster to a 5e monster, it just won't work in a formulaic way. The best thing to do is take the CR of the monster from 3.5 and then use the 5e DMG's charts formulas for creating a monster of that CR, using the recommended adjustments for offensive and defensive utilities. The best you could hope for is code that would take a monster's 3.5 CR and spit out its expected damage per round, hit points, AC, attack bonus, spell/effect DC with modifiers like "75% HP if >2 saving throw proficiencies" and sliders that would balance things like attack bonus and expected damage per round that have an inverse correlation to one another when maintaining a particular CR. You would still have to reverse engineer for what ability score modifiers, number/type of attacks, etc. to get to the recommended number ranges.
I'll repost my part of a PM conversation when Bhu consulted me for spell conversion.
On how to approach conversion. The context is to look at existing 5e spells as balancing points:
For buffs, you can go with multiple weak effects that combine into a strong combination (i.e. Haste), a strong effect with no concentration (i.e. Blink), a strong movement effect with a better movement type that requires concentration (i.e. Fly), a defensive aura effect with 1 strong bonus and 1-2 weaker benefits (i.e. Beacon of Hope), or an offensive aura (i.e. Crusader's Mantle). You can also downgrade effectiveness of any of these buffs for increased duration (compare Haste's offensive benefit to Elemental Weapon).
Haste is generally the most desirable 3rd level buff for offense. Balance any offensive buff against what Haste would provide. In other words, it should add about the same amount as an average weapon attack, independent of stuff like superiority dice and smites. If it adds more damage than that, don't give it extra benefits like Haste has.
Fly has both an evasive component and a speed component to it. Anything based solely on movement like Fly should make the character about as evasive. An example I could think of would be a spell that gave a burrow speed. Without debating the relative merits of each in specific situations, a burrow speed can much more effectively hide you from attack than a fly speed can, so a spell giving a burrow speed shouldn't be giving double speed to a character. If the spell only gives speed without giving evasion, it should probably be more than the 60 ft. Fly gives, it should not require concentration, or it should last longer than 1 minute.
Beacon of Hope's major effect is the advantage to Wisdom saving throws. Advantage to death saving throws is great for keeping character's from permanently dying but you're in a really bad situation if you're using the spell specifically for that. The maximum healing portion is kind of nice but healing in combat in 5e is about as good an idea as it is in 3.5, so it's a negligible bonus. I could see a similar spell that gave advantage to Constitution saving throws and resistance to poison damage. Poison damage isn't always applicable to an encounter, but when the resistance does matter, it's probably saving you from death more so than both of the minor effects of Beacon of Hope, so the theoretical spell would not need a second minor effect.
An offensive aura buff should be comparable to Crusader's Mantle. If it offers more damage, it needs to be limited in some way such as making it a once per turn effect for each buffed character. If it offers less, remove concentration, increase duration, or make it a bonus action to cast.
For aoe damage, a 3rd level aoe should average about 28 damage per target (i.e. Fireball, Lightning Bolt). If the average damage is lower, you can tack on an extra effect (i.e. Tidal Wave averages 18 per hit but knocks prone). If the damage is significantly higher than 28 per target, it should probably deal no damage on a successful saving throw or be an easily resisted damage type like poison.
Specific examples Bhu asked for:
Keen Edge
3rd level transmutation
Casting Time: 1 bonus action
Range: Touch
Components: V, S
Duration: Concentration, up to 1 hour
You touch a nonmagical weapon. Until the spell ends, that weapon becomes a magic weapon that critically hits on a roll of 19 or 20.
At Higher Levels: When you cast this spell using a spell slot of 6th level or higher, the weapon critically hits on a roll of 18 to 20.
Divine Favor is a 1st level spell on the paladin list.
Divine Power doesn't translate too well to 5e since proficiency replaced BAB and everyone has the same one. If I were to make it 3rd level, I'd probably make it a transmutation and base it on Haste. Haste replicates Expeditious Retreat and Shield of Faith while providing a unique bonus, so I took 5e Divine Favor and Heroism for the minor effects of Divine Power.
Divine Power
3rd level transmutation
Casting Time: 1 action
Range: Self
Components: V, S, M (a holy symbol)
Duration: Concentration, up to 1 minute
Calling upon your divine patron, you imbue yourself with strength and skill in combat. Until the spell ends, your weapon attacks deal an extra 1d4 damage. When you take the attack action, you may make one extra attack. You gain temporary hit points at the start of each of your turns equal to your spellcasting ability modifier. When the spell ends, you lose any remaining temporary hit points from the spell.
5e Magic Weapon has scaling built into it, so I'm not sure what Greater Magic Weapon should do.
Righteous Might needs to combine the effect of Enlarge, Shield of Faith, and multiple-turn Blade Ward. That would be really hard to justify as a 3rd level spell. The conversion wouldn't be too hard but it would be a 5th level spell at its weakest. The original PHB version (as opposed to the SRD version) would be 7th or so.
Most monsters that used to have spell resistance now have advantage on saving throws against spells and magical effects. Paladins have a 5th level aura spell, Circle of Power, that does that with a 10 minute duration. If you want a 3rd level version of that, it would probably look like:
Spell Resistance
Casting Time: 1 action
Range: Self
Components: V, S, M (a holy symbol)
Duration: Concentration, up to 1 minute
Until the spell ends, you have advantage on saving throws against spells and other magical effects.
Spell Immunity is kinda replicated by Globe of Invulnerability in 5e, a 6th level spell. A 3rd level version would basically be Globe of Invulnerability scaled down to block 2nd level or lower spells. If you wanted it to move with you, it would probably make it one spell level higher, so a 3rd level version would only block 1st level spells and cantrips.
True Strike is a (crappy) cantrip in 5e already. The spirit of the spell in 3.5 is basically to not miss. The closest effect to "never miss" I can think of is the ranger's Lightning Arrow spell, which is a bonus action buff to 1 weapon attack at 3rd level. Lighting Arrow can miss but still deals half its damage and its splash damage on a miss. That wouldn't be hard to create something with since it only takes a few wording changes.
