So, if you are a modder of repute you can apply to Beth to receive an advance copy (tied to iron-clad NDA) of the construction kit to begin modding for Skyrim before it even launches. Moders like Slof and Robert likely already have them, if they are interested.
As a result, and guessing by the community as a whole I'd say the first mods off the rank will be:
A variety of nude mods to remove force-welded undergarments and allow characters to strip down naked etc.
Along with this, body and skin optimisation and improvement for the base races, to make them prettier than Vanilla.
Clothing and armour mods using remodelled and reworked game resources, followed swiftly by original armours and clothings.
Next out will be a slew of housing mods, likely neck and neck with a variety of companion mods.
For each of these categories, there will be a brief dominence battle before one main method (or a few) takes a lead. In oblivion, most companion mods ended up using the CMpartners code, for example.
A huge splattering of race mods, from the smooth and delicate, to the recoloured-naked-elf-babe, to the grosely ill-fitting and/or overpowered, will also begi to show up inthe first few weeks, but will grow at a steady rate throught the game's lifespan.
Several custom races will become dominantely popular, and a vast array of re-works, options and clothing and armour customisations for them, built off or using said race mods as a base, will arrive.
After the inital explosion of character- and personal-centric creation and modification, we'll begin to see the world-redux mods, to transform the assest ofthe whole game one way or another, from as simpl ea thing as re-optimisin g and prettying up thigns, better environmentals, better-x -y -z, through to Skyrim-as-a-victorian-neo-classic-steam-tech world type mods.
About this time we'll also begin to see the RP related and immersion mods coming intot heir own in greater force: the realsitic living, and realistic world type mods, such as the needing sleep, needing food sort of variants.
....
Personally, however... I'm really hoping someone is working ont he next generation of Mog-mod or other moogle-race mod, since that's the character I want to play, heheh....
-Harla