Right now I'm as interested as the next person who wants to know where they're going with the engine. My two big hopes, that seem realistic enough, are a 64 bit exe with DX11 support. Both console platforms are already set up that way so there'd be no logical reason not to do it.
As far as engine bugs, the biggest one that got introduced into Skyrim with Patch 1.9 is the lip sync bug. In severe cases of that, NPCs have lip movement that makes things look like a bad Kung Fu movie dub. We've got some other irritations that came up that have been listed here: http://afkmods.iguanadons.net/index.php?/topic/4144-skyrim-engine-bugs/- the most annoying of that bunch being on post #6.
Whether or not I mod the game depends on if it holds enough interest. That didn't really happen with FO3. I made all of one mod before deciding it wasn't for me and went back to Oblivion. The game was cool, I just had no interest in dealing with the navmesh bug, among other things. Didn't really care much for NV so never got involved with anything there. I haven't even done the DLCs for that.
I honestly wouldn't count on FO4 being enough of a grab to have any desire to fix a bunch of bugs either. Especially not this close to Skyrim. Skyrim's unofficial patch project is becoming quite a draining experience and I don't want to be piling on a whole new game while we're still in full swing fixing Skyrim bugs after all this time.
Since the subject was raised, I'll also state I would like to see paid mods come back for FO4's official launch, and I would also like to see them come back for Skyrim too. I don't think the program was given a fair shake by those who were against it. Angry mobs should never decide the fate of a project.