I think there is a lot of scope to tighten up PC building - eg fewer (or less fiddly) feats, reducing the dependence of combat stats on ability scores, a less clunky de-integration of magic items than inherent bonuses.I'm curious what 4e fans would consider an improvement of 4e.
The trick would be keeping the resulting mathematical output more-or-less consistent with the current maths base of the game.
And one aspect of the game which can't be fixed while holding the basic maths constant is fixing the discrepancy between scaling of combat bonuses/DCs and scaling of skill bonuses/DCs. (Which is one of the contributors to the sometimes irritatingly clunky interface between combat and non-combat resolution.)