What I intend to do with 4e right now (unless somehow wowwed):
Examine the combat engine and see if there are any imporvements to be made on my houseruled 3.x.
What I would do with 4e as a 3.x product:
As above, plus:
Consider the rammifications of PC core races that are not humanoid. Elves as fey and dwarves as giants are part of my houserules, and it seems that 4e might do some good in that direction.
A gentler power curve, lower-magic version of 3e....rejigger the math so that attack bonus, saves, and AC don't become so extreme. Reexamine spells, putting in some of the penalties for some spellcasting that used to be in the system. Make item creation as accessable but not as universally applicable (in my homebrew, the Item Creation feats give you X number of item types you can create, which increase with level or by discovering "Secrets", i.e., formulae).
Rename the Feywild and the Shadowfell, and use them with 3e.
Consider the Quickplates in Monsternomicon as an ideal level of work to quickly differentiate monsters, and go from there. Keep the potential complexity. All templates should include all information needed to alter a monster; they should not require you to look up charts elsewhere.
Weapon skills. I've posted my system on EN World, and it is pretty easy to use, offering tactical choices without overly slowing combat down.
Ditch the AoO list; shortlist it to "retrieving item, casting spell, making ranged attack". EDIT: And Move-through occupied space, enter reach space with weapon set against charge.
The idea of recovering some damage by resting is a good one; use it.
I'm sure that there is more, but that's what strikes me off the top of my head. Basically, IMHO, create a "version" of 4e that is an actual revision of 3e, rather than its own thing. If there was "bridge" compatability, allowing both 3e and 4e materials to be converted fairly easily, that would be ideal.
RC