Fiora's wouldn't be too bad, if you wanted to go unkillable tank. Massive amounts of stacking regeneration.
Nasus? Heh. You could easily stack over 100% lifesteal.
Nocturne? Lifesteal some more, just in an AoE.
Lets not get started on those dirty tanks. Rammus with a Thornmail would be one of the best AD carries in the game, similarly for Galio and a bit of MR.
Ideally, I'd like to have a stealth champ so that when a game goes bad, and the enemy has their foot on our collective throats, I could slip past, get deep into their territory and suddenly be all "O HAI im in ur base killing ur towerz", to draw some of their folks away from the group push.
Of course, that would require a fast ubertanky stealth with a lot of burst damage that could be targeted on a tower. So I fully expect Riot to make one soon.
It would be interesting to see a champion that had to be either a hard tank or a carry, with their abilities scaling against one another... say, if you stacked more health/armor/MR/etc, your skills would benefit more defensively, whereas if you stacked AP/AD/etc, you would be unable to benefit from said defensive abilities, but gain more of an offensive advantage.
For example, if you had some form of stealth, the duration of the stealth would increase if your bonus health was higher, but the steroid (because you can't have stealth without a steroid) would be better without it, with more health impacting the steroid more negatively. Then, a form of AP nuke that has better defensive results if you build MR, and inversely does more damage if you build pure AP. But not both, for any of them. Similarly, an AD effect that acts as the opposite of armor.
It would deny being a pure hybrid, but make up for versatility when you strive for a specific item build. A true "jack of all trades" character. Build bonus health, AP, and armor, become a stealthy gank nuker who is actually hard to kill by AD carries. Or go pure assassin, and get nuked into the ground after you blow your load.
Probably would be hell to balance too, which is why I fully expect Riot to not do it.