I'm part of a project at the WoTC forums to create a Warhammer Fantasy D20 setting. I was recently thinking of "branching" into Planescape by using the "Clueless Accident" setting- aka the "bunch of Clueless accidentally stumble into a portal to the Planes" basis.
The problem is that I was intending to use an adventuring band composed of Warhammer D20 characters, and this raises a few problems because Warhammer's cosmology is so different to that of "conventional" D&D. While to a degree this can be explained as simply having their Prime be a particularly obscure and cut-off one, there are still some problems, which I'll outline below (at least, the ones I can think of). Can anyone suggest a way to "work around" these problems, so that they don't render the party too powerful?
*Problem one: Warhammer spellcasters get essentially unlimited spells per day, albeit with the fact that whenever they cast so much as a Cantrip there is a chance the spell backfires. This can have effects from inflicting around 8D6 damage to driving them mad to erasing them from existence altogether.
*Problem two: Warhammer Dwarfs -that's how the plural is spelt in this setting- are resistant to arcane magic at the cost of being unable to take any levels in spellcasting classes.
*Problem three: one or more of my players is likely to want to play a Chaos Champion- basically an Evil Paladin who transforms into either an insane Aberration or a Fiend at level 20. While these end fates are a ways off, there are other problems- primarily that they get a random "reward" at each level. These rewards can take all manner of forms, from razor sharp claws, rending fangs or venomous stingers to (involuntary) teleportation/ethereal shifting, acidic spittle or fire breath. And that's not mentioning the unique ability of each subclass of Chaos Champion...
Here's how I'd deal with it
Problem 1 - Don't change a thing from core rules. Warhammer mages have enough spell slots to last a 6 turn battle. This isn't unreasonable for any decent mage although the Warhammer mage limited spell selection and freedom to choose their spells is more similar to a sorceror than a wizard.
I'm sure you could make a prestigue class (mixing in aspects of wild mage I imagine) but I'm not sure it's worth the bother
Problem 2 - Subrace. Give em 15-20 magic resistance and ramp up the LA. Barely anyone wants to play a Dwarf mage anyway and if they really want to they can go with the standard race and call em a mutant/chaos dwarf.
Problem 3 - No problem but some work. Chaos champion is just a Prestigue class which some chaos warriors take. Make a table of weird abilities for them to roll on each level and have spawndom at the end if they haven't become a demon prince by that point. A somewhat more elegant solution would be for them to lose 1d4-1 wis/int per mutation and once they reach 0 they become a spawn.
As I say that's just how I'd deal with it and my knowledge of Warhammer is a tad dated (about 8 years) so feel free to take or leave what you like.
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