Nasus is a champ i dont enjoy playing against. Same with Gangplank, Teemo or Singed. They have one thing in common, you cant really trade vs them; they are uninteractive. Since Nasus got buffed recently his playrate went up and because people dont like Nasus, so did his banrate.
The idea behind stacking up until youre a Raidboss is not bad, i actually like that. Let Nasus keep his idendity of stacking Q. The real problem is that most ad-buiser toplaners relying on autoattacks can not trade with nasus, because of the Point and Click instant Slow, which cripples your attackspeed, to the extent where it feels like being stunned for 5 seconds, while champs like Lissandra or Renekton can still destroy him.
To make league less counterpicky my proposal is to change his W into a skillshot, that slows the enemys attackspeed (not the movespeed), make it dodgeable similar to morgana Q, and buff his Q to give him 4 stacks per lasthit or compensate some other way.
That way laning vs nasus feels interactive, because you can then win trades because of skill, not because of champ select. If you want Nasus as your Raidboss you should build around him anyway, so with a CC team this new faster-stacking nasus can be relevant late while not being oppressive in lane.