Are there any items people can think of that are percentage on hit rather than /day? Perhaps these can be used as a guideline.
Only Vorpel comes to mind and from there you actually have a decent idea. For instance we can derive a 5% auto kill vs most monsters is equal to the scaling cost of a +5 enhancement bonus. Here is a broken down thought train to consider with some suggestions.
1.
So what else is nearly auto kill? Damage of course. Like a Pit Fiend has 225hp, Balor 290, Solar 209, TDs kind of range in the 400ish area but lets name a value lower than that and call those out 'miss' window like headless creatures. So an effect that deals around 200~300 damage is likened to auto kill. This approach is also better comparable and more likely to appeal to an MMO Player (moar DPS!) then trying to figure out save or suck debuffers as well.
2.
If we further type cast it we can limit the window a little bit more and even branch out a verity of abilities. For example there are five basic energy types; acid, cold, electricity, fire, and sonic, or even the three special ones; force, negative, and positive. While ultimately we have a series cycle of vorpel rolls that deal 250ish damage, they can be flavored differently and further tweaked based on the elemental concept. Likewise there are more creatures immune to Vorpel than there are none-epic creatures with more than 250hp so the elemental addition brings resistances and immunities into play.
3.
Bottom line is it needs to feel special. This little effect needs to
feel special, Vorpel's "I rolled a 20, meh he dies" is nice but telling the player to roll a handful of dice is even better. A tad slower yeah, but it feels more AWESOME! Like wise you would need enough varency between the elements to attract attention. Like the one dealing fire damage sets them on fire, or better yet covers them in lava!, while the cold one doesn't do coated in ice.
4.
So stealing some of DDO's name flare, here is three suggestions to consider.
Crushing Wave: Upon a roll of natural 20 and confirmed critical hit this weapon releases a torrent of water from the deep sea that surrounds the subject hit by this weapon. The water is both fridge and heavily pressurized dealing 5d8 cold and 5d8 bludgeon damage each round for five rounds. ~ avg 225 total, 45 per round which may prevent spellcasting.
Lightning Strike: Upon a roll of natural 20 and confirmed critical hit this weapon emits a powerful lightning bolt dealing an additional 10d6+200 electricity damage to the subject. The effect is loud, providing a +20 bonus to listen checks to detect the battle, and bright illuminating the battle field as if a torch were burning in the area of attack for one round.~ avg 235 done in a single round.
Magma Surge: Upon a roll of natural 20 and confirmed critical hit the subject hit by this weapon is covered in lava for two rounds. Treat them as if they were fully submersed in lava while covered by this effect, dealing 20d6 fire damage each round and an additional 10d6 for 1d3 rounds after this effect ends. ~ avg 192ish total, 45 per round which may prevent spellcasting.All effects are considered a +5 enhancement requiring the crafter to have a CL of 18 and any one 9th level spell with the appropriate descriptor.
5.
And finally, damage is a scaling effect. Scaling here being an operative word. If you could draw a line through the enhancement costs, and I'm ballparking this but if +3's price is generally half of a +5er then you can halve the damage or so and create a 'lesser' version for lower level characters to pick up earlier.
