I won't start modding until I finish my first full playthough at least.
Then, probably my first mod will be to tweak a weapon that I like aesthetically but whose stats are crappy. Or a nice custom house/castle if I can't find any already avaliable.
Helm's Deep anyone?
If the engine and AI are massive battle-enabled, it'd be cool. And in PC's the Skyrim executable could be patched in order to use 64-bit memory allocation, so here you have it...