I'll focus my comments on PC building.As a general question what things from 4e would you (general you) have liked to have seen improved, explored more in depth, or expanded?
A pretty easy thing to do would be to have more scaling powers, thereby reducing bloat and making options clearer. CaGI/Warrior's Urging is the obvious poster child for this, but it's not the only one.
The next step would be to adopt a more "Essentials" approach to certain classes and their powers - ie group availability under some larger headings. I think some utility powers might particularly lend themselves to this - and perhaps some rogue utility powers should really be skill powers.
This requires some relabelling of options, but nothing fundamental.
The fundamental change I would make in PC building, but it would be very dramatic, is to drop stat as a component of attack. (Leave it damage only.) All attacks could then be put on a uniform base. You could build enhancement bonuses for items into this too (so, again, they would contribute only to damage), and expertise feat bonuses.
The final step in this process would be to bring attack bonuses and skill bonuses into rough alignment, so that the combat and non-combat systems integrate more smoothly (which they clearly fail to do by the upper levels of Heroic - MV had to change the grapple escape DCs in recognition of this!).
Cleaning out redundant feats and systematising magic items would be obvious housekeeping as part of this project, but sorting out the maths would be at the heart of it.