If you ever played Final Fantasy VII, then you'll remember Ruby Weapon (or to a lesser extent, emerald weapon). It took me months after i beat the game to take him on. The Final Fantasy's have done this with a lot of their games and i think it's something Bethesda should consider.
Maybe some Dragon in a huge underground cavern, that doesn't level scale, and it takes a super-high level character to beat, with some reward that's really worth it.
Or a master swordsman, who lives on top of one of the largest mountain who is seemingly impossible to best.
Something along these line I would love in Skyrim, or at least one of the expansions.
YEAH!!!! thats a great idea, man i loved the challenge of emerald and ruby weapon, i used knights of round combined with HP absorb, with mime, and if you had another character with mime even easier and faster, ruby however, the best way to beat him was to enter the battle with all other teammates dead, and i think it was final summon, combined with life2 and your practially invincible, so yeah a super insane adversary would be a perfect addition to this game, because once your all powerful every enemy is just to easy and boring eventually, ya know what would be even cooler? is if they added something like the monster farmer from final fantasy 10, so you could have a vast variety of god-like adversarys to battle taking it to the next level for your challenge with massive bonus's and payouts, but they should only add such a feature after the MQ has been completed so theres a huge bunch of extras to do so the game doesnt really end.