Right so. I think we should go about this by SRD chapters.

Before we go into details, the basics.

Do we have to change anything about the basic framework of the d20 system?

This includes for me:
Task resolution mechanic: d20+bonus-penalties vs. target value.
Six ability scores, 10-11 being the basic value, negative and positive modifiers, bell curve distribution between 3 and 18 for humans, etc.
Hit Dice.
Base Attack Bonus.
Feats and Skills (not what they do, just how they are gained).
Action types: Free, Immediate, Swift, Move, Standard and Full round.

Anything we need to change about that?