combining d20 systems (3.5e, future/modern sw etc

[Rodrigo Istalindir]

This is why we have Grim Tales. It will give you a d20 Modern-esque base for your characters, but is flexible enough to handle most D&D stuff. You will find you have to tone encounters down a bit since the characters won't have access to magic items and especially magical healing. But the book does an excellent job of helping you tune the rules to meet varying levels of lethality.

 

[Garnfellow]

Every time this topic comes up, I am compelled to point out that Second World Sourcebook analyzes the differences between D&D and d20 modern in great detail and provides a variety of strategies to deal with the differences.

Yeah, the Second World discussion of mixing d20 M with d20 fantasy is pretty darned good. I have no interest in the Second World setting itself, but for my money, the design notes were worth the price of admission.

As noted above, class defense bonuses and Massive Damage Thresholds are probably the two biggest things to deal with.

 

[jmucchiello]

The big problem I see is that some/most d20 versions give a Defence Bonus by level, while D&D doesn't. If you let D&D magic item AC bonuses stack with other d20 systems' level-based bonuses the D&D classes will be underpowered relative to the other ones. Possible solution is to give D&D classes their own level-based AC bonus (eg Judge Dredd uses the Ref save bonus as AC bonus, that'd work) while keeping magic item AC boosts rare.

Better probably to not allow defense bonus to stack with magic enhancements to armor bonus. Thus the d20M character has his defense bonus and cannot increase with a ring of protection. The D&D character has no defense bonus but can utilize the rop.