Laslo Tremaine said:
True20 is pretty much the perfect game system for me. It hits almost every note just right. Unfortunately, for this specific purpose, I want easy compatibility with existing d20 material, and mainly because of the Damage Save mechanic, True20 complete falls down.
I don't see how True 20 completely falls down here. The True 20 appendix contains comprehensive conversion material that allows you to change the damage system in True 20 to D20 and vice versa very simply and easily.
Using the appendix makes convertin d20 monsters a breeze. It takes me no more than a few seconds to convert a D20 monster to True 20.
If you want to keep a D20 damage system in True 20 instead, it isn't hard to do so and this removes the need for any conversion other than PCs:
Toughness to Hit Points - Adepts get a d6, Experts a d8 and Warriors a d10.
Damage bonus to Damage dice - +0 is 1d3, +1 is 1d4, +2 is 1d6, +3 is 1d8, +4 is a 1d10
Healing - Use the existing D&D rules for HP healing. True 20 deals with converting healling magic in some detail.
Armour bonus - I would convert Armour bonus into a DR dice system like in Iron Heroes, using the Damage conversion system above.
What exactly do you find so challenging about the conversion?