I think it'll be something simpler than any body mod - people will need time to see how the new bodies are, how they're articulated, how they interact with armor & clothing models in the new engine, etc....
Meanwhile, I'm sure someone will get annoyed by some small thing, and immediately make a mod to fix it. (Like, turning off quest markers. Or fast travel.)
The "I made this cool magic sword by taking an in-game model and giving it HUGE magic bonuses! It's in a chest right where you start!" mod is also pretty easy and quick to make. :rolleyes:
----
Edit: just checked Fallout 3 Nexus. FO3 came out Oct 28. The first mods, on Oct 30, were "no quest markers on compass," "no quest waypoints on map," and "no NPC markers on compass."

Edit2: and TES Nexus. Oblivion came out March 20. March 22, there was a "no fast travel" mod.

(it was second... first was a portable trashcan you could use to destroy junk you didn't want? Odd.)