I would quite like a custom music mod (and custom title screen option) similar to the ones for F3/Oblivion, i am not suggesting Skrim's are bad (on the contrary) i just like messing with these things.. http://www.youtube.com/watch?v=PK7kmRNfi4s (my candidate for menu/title music)
As i mentioned in another thread i would love a Severance BOD inspired combat mod (limb removal & all) to make the melee combat that much more visceral/risky, but with some bigger visual feedback (but not overused as in F3/New Vegas etc)
Dynamic fire/spell effects on wood,grass/general foliage etc would be great too.. ie: enemies caught in burning grass etc would also take damage..
I have plenty more ideas, but will stop here..
What would you like to see..?