Skyrim releases officially on 11.11.11, but modding discussion and planning is already in progress. Here are a few mod ideas currently being bandied about in the community:
Skyrim Arena Mod
Skyrim won't have an official arena, but many players who went through the gauntlet of the Arena in Oblivion want there to be one. (Oh no, Elf twins!) An arena mod may very well be challenging, but discussion of a potential arena mod is already underway in /r/skyrimmods so watch that space, and offer assistance if you can!
Skyrim Weapon Mods and Skyrim Armor Mods
Of course weapons and armor will be up for modding, and with Skyrim's new inventory system that allows you to inspect items in detail, expect the scope for creatively made items to be absolutely huge. In the past, spending hours pouring over the detail of a scabbard or hilt didn't really make much sense, but Skyrim rewards those with attention to detail.
How Do I Make My Own Skyrim Mods?
A good modding creation and tutorial pack is available for Oblivion here. Oblivion is not Skyrim of course, but learning how to mod in Oblivion is going to introduce you to the basics of modding and give you a head start when Skyrim arrives.