Combat. D&D has problems with nonmagical combat. For one, it is relatively inflexible compared to magic, though that is also largely a problem of the classes that use it.
This thread is for the basic, underlying combat system. A few points to think about:
-What shall we do with combat special attacks? Should they be simplified? Should there be more of them?
-What about full attacks? They make mobility in combat difficult, and the last few iteratives at high level are often almost pointless, as a -15 to hit compared to your main attack will rarely do much.
-Should we include a maneuver system by default, as per Tome of Battle?