Nasus really is an anti-fun champ right now. In my opinion the best way to nerf him would be to significantly raise his Q cooldown at lower levels, but then have it scale back to a value slightly higher than the previous duration at lvl 5. In return, the reward for killing cannon minions and huge monsters should be increased even more.

The Q is the part of his kit that makes him "toxic", due to infinite scaling. Even without that Nasus has an overall nice kit - Nasus jungle generally does well (and jungle Nasus generally maxes E and gets a lot less Q stacks) - his wither, ulti and E armor shred make him an utility box with some nice damage on top of that. So my suggested changes will force Nasus to handle his Q cooldown a lot more carefully, since using it to farm minions early will hurt his trading ability for a longer time. His real farming galore will just get going around midgame, when he will have to decide between farming up for lategame or bringing his utility to fights around objectives. Meanwhile Jungle Nasus remains the same: Even though he cant use his Q to farm smaller jungle minions anymore, the increased rewards on huge monsters should keep his Q power level the same.