Nasus is a bit of a pain to balance. He's already got late game carrying potential.
(Key word. Potential. Please do not crucify me with "JUST LOCK HIM DOWN, YOU MORON" comments.)
One small change and you could end up with the current iteration of Veigar, whose ult is essentially Garen's with Magic, and scaling with LOOKING at enemy champions. (As someone who loves Veigar, I'd rather see his scaling put back to AP on Champion Kills. Taking advantage of his early farming phase was a key point to deal with.)
Buffs? Probably not going to happen unless with a complete kit rework. Even then you might end up with a Poppy-Like Q.