On Multiclassing and Domains:Shugenja is a fairly flexible class, given that the various Elemental and Order foci allow for the emulation of various other classes. That said, the only benefit to advancing Shugenja beyond 1st level is spellcasting; for many optimizers, that's a clear indicator to get out of Shugenja as quickly as possible, into another base class or PrC. If you choose this route, here are some thoughts on the various benefits:
Barbarian:Get some extra HP, weapon and armor proficiencies, and the ability to froth at the mouth
instead of casting spells. This sounds like a recipe for trouble. A minority of Earth Shugenja with good physical stats, and who are emulating a gish build,
might find utility here in a one or two level dip.
Bard:With no change in primary stat focus, gain additional skill points and a whole lot more skills, along with the bardic music goodies. Two levels here add much to your versatility, including a gateway to that juicy Sublime Chord progression.
Cleric:A single level dip into Cleric nets you that coveted Turn Undead ability for DMM fun, as well as access to a couple of Domains.
However, it won't let you progress Domain casting as a PrC would, and requires that you spent on what is otherwise your dump stat. Fire Shugenja might consider it for the access to healing.
Druid:The Animal Companion will be stuck at low levels of effectiveness, and you'll have to allocate WIS for the spells. You'll also probably be getting out before you can Wild Shape. Fire Shugenja might consider it for the access to healing.
Fighter: A single level dip - maybe two, no more - gets you considerable benefits if your Shugenja regularly finds himself in the thick of the battle. Those bonus Feats can help Earth Shugenja especially, as the ones most likely using attack oriented spells.
Monk: Emphasis on your dump stat and physical confrontations, for which you are otherwise ill-equipped, in order to do less damage than a well-conceived 1st level spell. Pass.
Paladin: Assuming you can live with the PITA code/alignment issues, this is arguably the best choice for a martial Shugenja. Two levels here will do you and your party a world of good.
Ranger: A small HP increase, some off-focus skills, and a combat style you're probably minimally equipped to use make this among the less attractive martial options for a Shugenja unless you're facing hordes of a specific enemy. It's better if you have access to the variant Ranger fighting styles from Dragon magazine.
Rogue: A wide variety of social skills - along with UMD - with plenty of skill points, Evasion, and a small, reasonable boost to your combat prowess make a two-level dip into Rogue a very tempting idea.
Sorcerer: Single attribute focus: check. Spells: check. Lots more versatility: check. If you go this route, you're aiming for a Theurge PrC of some sort.
Wizard: For a CHA-focused build, Sorcerer is just a more sound option. If Scribe Scroll is this important to you, buy the Feat.
Domains:Since I've sung the praises of Domain access throughout this handbook, naturally, the following retort came up:
Then what domains would you recommend ? (Don't tell me you didn't see that coming. )
Well, alrighty then.
Air - Very cheesy way for an Earth Shugenja potentially to gain some otherwise banned spells for more battlefield control, and a turn undead clone ability.
Animal - Extremely minimal gain here.
Death - Domain ability is likely underwhelming for your Shugenja, but a Water Shugenja becomes much more combat-viable with this one. Some redundancy with Fire.
Destruction - Possibly even better for a Water Shugenja than the Death Domain, this gives you a better attack option and lets you do your healbot shtick with a necropolitan party member.
Earth - See commentary on Air Domain, and apply it to Air Shugenja instead.
Fire - See commentary on Air Domain, and apply it to Water instead.
Good - +1 caster level on a small subset of your spells, and some decent buffage along with a couple of quite nice higher level spells. Earth Shugenja will find similarities.
Healing - See previous commentary on the 'benefits' of being a healbot. Could be useful to a non-Spellscale Fire Shugenja.
Knowledge - If you're the only diviner-type in the party, it COULD be useful, maybe. Probably not.
Law - Quite a limited sample of spells to boost with the Domain ability. Calm Emotions and Hold Monster are nice, though.
Luck - Rerolls are handy. The spell list is likely duplicated at least in part by extant spells on your lists, though.
Magic - Very nice Domain ability. Spell list pretty closely mirrors the Ineffable Mysteries.
Plant - Quite a few useful battlefield control spells here to go with your faux turn undead.
Protection - Useful for spell access throughout, especially for Air Shugenja. The Domain ability will potentially go unused.
Strength - Greatly expands an Air Shugenja's options for party buffing, and adds an otherwise forbidden buff for Fire Shugenja. Somewhat off-flavor for a Shugenja's standard archetype.
Sun - Spell overlap with Fire domain with slightly less cheesiness for a Water Shugenja. The Domain ability is useless without Sacred Exorcist or similar.
Travel - Freedom of movement ability and a potentially handy new class skill - if your DM isn't overly literal - make this attractive. Spell list is largely duplicated by the Air Shugenja list, with D. Door as a nice perk.
Trickery - A CHA-monkey like the Shugenja should enjoy the new class skills. Earth Shugenja benefit most from the spell additions before 8th level.
War - Earth Shugenja will gain virtually nothing here unless they want the MWP/F. Air Shugenja worshipping a martial god will likely benefit most.
Water - See commentary on Air Domain, and apply it to Fire instead.