Nasus should be a late game champ, and here's one way to do it
Give his ult a passive along the lines of:
For every 100 stacks, gain 1/1.5/2% base armor, mr, and base AD
The scale would be great since Lord domis grants nothing against this passive and his sheen Qs continually get stronger.
Besides, his base stats kind of suck compared to other juggernauts, so this would be a good way to reinforce the idea of him being that late game hyperscale juggernaut. His tank stats are mediocre and his base AD is an absolute joke. Something like this would truly fit into nasus becoming that imposing juggernaut that you have to stop before he scales into oblivion.
Nasus should feel like the ticking time bomb that you have to defuse, not the mid game potential-horror and late game laughing stock.
EDIT
the % scaling is ONLY OFF BASE STATS. It's not going to be massively oppressive people, its just something to really reward nasus for getting to that point of stacking. There is a reason champions are given infinite scaling, and its not for them to be useful early or mid game...
or 
who, just as a reminder, has the most powerful single target slow in the game, on an 6.6 second cooldown.