Let's be honest: Nasus' Q is broken, and though what I'm about to say might also be applied to Veigar, the difference between these two is that while Veigar is a nuke champ who has to back off between engages, Nasus' kit enables him to sustain himself in a teamfight where the odds are stacked well against him and still deal massive amounts of damage.
The problem with Nasus in general is that he dictates the flow of the game from top lane. The only time Nasus is decent is when his Q is stacked, so Nasus players will always stay in top lane and keep stacking regardless of what's going on elsewhere to the point that you must dedicate a sizeable amount of time to defending top lane instead of pushing the other lanes, otherwise Nasus, while stacking, will keep pushing your top lane back till he's taking down your inhibitor. There are several issues with Nasus' Q that make him a terrible opponent and some of them synergise with the rest of his kit.
For starters, his Q scaling exponentially is a bad idea. Sure, you don't expect a match to go on so long that he can build enough up to one-shot champions but even then a stacked Nasus can pretty much do the job after 15 minutes anyway, especially with more vulnerable squishy champs. But the problem there is that his Q scaling is basically what Nasus is all about, so to take that away from him will destroy his image, his playstyle etc. and that just isn't something Riot (in my opinion) are prepared to do. (Because fans) Then you couple that with his armour-shredding AoE to make sure that what little resistance you can put up against his Q is hampered further, plus a targeted movement speed slow that increases to 95% over 5 seconds to make sure his target doesn't get away. On top of that he has an innate tankiness and an ultimate that _increases his attack power_ while giving him extra reach and extra health, helping him dive into the middle of the enemy team, shrugging off the damage and zoning in on the enemy carry to take them out in one shot.
No champion should be able to get so tanky and deal so much damage by themselves, especially not when they only build tanky items. I suggest limiting the power of his Q to stem his lane-bullying nature and help bring back a 'Nasus' game into a normal, diverse game. For starters we could make Nasus lose stacks on death. Many other abilities that require stacks fall off or are lost, the same for certain items too, so why not Nasus' Q? I think as the rank of his Q increases the percentage he loses on death should also increase, to stop him dominating team fights in every game. This means that he doesn't lose his characteristic OP-ness but that the usual gung-ho playstyle of Nasus' comes with a cost other than having to wait to respawn.
I also suggest that his Q should have a longer cooldown if it fails to kill a target. This means that if he initiates a champion with it but doesn't kill them he won't be able to simply use his W and wait four seconds till he can finish them off with another. It also means his stack-building isn't hugely affected because so long as he times it right the short cooldown will remain. This also allows him to mop up in team fights so his power is still present.
Lastly I propose is to reduce the damage his Q does to towers. As I said previously, any game with a Nasus in it will veer towards the top lane because a good Nasus player will keep stacking and pushing as the two go hand-in-hand and will eventually pave the way for OP Nasus to take his place. But if his Q did reduced damage to towers it would slow him down enough to limit his inevitable impact and perhaps encourage Nasus players to join in with the rest of the teams. Couple that with the extended cooldown for non-lethal hits and you have driven back the endless top-lane Nasus push.
What I wanted to do here was to help reduce the dominance that Nasus has in any game he appears in. Champions have their day but Nasus just seems to keep being a massive thorn in everyones' side. Sure, with coordinated CC he becomes only a nuisance but the lower you get in the leagues the less likely you are to find it. Some might say that he has been optimised for professional play but no one plays Nasus professionally because, as I said, with coordinated CC he never wins, so what's stopping Riot from bringing him in line with a hobbyist's skill level? I just don't want to play a game against him knowing exactly how the game will run. I want to be surprised and challenged, not faced with the inevitable.
This is one man's humble opinion and, likely, a flameable one. So please, be gentle. I'm just looking to improve the game for everyone.