Bad Romance could've been a ballad, I like the stripped down version. The lyrics are also rather ballad-ish if you forget they go along with one of the best productions of all time
A ballad that could've been an uptempo is... on second thought I can't think of any.
That note at 2:09 is enough life to birth all of Octomom's offspring.
And they said she couldn't sing.
I'd have to say Disturbia and Paparazzi are my two favorite uptempos turned ballad. I honestly thought HIAM would sound awful, but I actually really love that cover posted above. I may need to download it.
Also, the Chew Fu Black Russian remix of Russian Roulette makes me thing it could've been a good uptempo, too.