I expect the first mods to be minor tweaks to any features that people have a negative reaction to... but I'm just making that up. To be honest, I'm not even sure if the body replacers will take that long to show up, but I'd imagine it would take the fans with 3D modeling skill time to study the default models and decide just how they want to model modifications. It's not just about dirty stuff, and they do usually include versions with underwear. Then again, perhaps there is a segment of poorly done nudity mods that get rushed out at release and are long overshadowed by more serious body modification mods later? I will say though I think it's hilarious how female mods are so much more numerous and quick to appear than male ones though. I suppose that's only to be expected when most gamers are male.
As others said though, I expect the first popular mods to be tweaks to stat or perk systems.
I wonder how hard it will be for people to modify the UI. If I recall someone from Bethesda said it outright uses Adobe Flash. Does that mean modders that change the UI will need expensive editing software? I suppose there might be open source alternatives.