As someone who's played quite a bit of Nasus, I think he needs hit power shifted a bit. The 12 stacks he gains per large monster or minion is too strong in games that are lower in elo, where players can't co ordinate as well to kill a Nasus with a lot of stacks, while it is weaker in higher elo where players are far better at kiting, fighting and eliminating targets with high damage and no mobility boosts. I think it would be wise to nerf the stacks gained overall, or mix how much stacks he gets on monsters and minions per leveling up the Q to slow it down a bit in the earlier levels. Example:
Q1: 1 stack per monsters and minions
Q2: 2 stacks per minions and monsters
Q3: 3 stacks per minions and 3 for monsters
Q4: 3 stacks per minions and monsters and 6 per large minion or monster
Q5: 3 stacks per minion and monsters, 12 for large minions and monsters.
Now if we did something like I mentioned above, the problem comes into his early game, which would be almost impossible to play until level 9 or so, where he maxes his Q, and would slow down the rate at which he stacks until around 15-20 minutes in, hurting his lategame scaling. To make up for it, it would be a wise idea to increase his Base health, Base health per level, base health regen and his Base AD. This will help him with his lane sustainability, helping him stay in lane, or jungle, longer, thus soaking up more exp, help with his early trading, and help him last hit minions better. Increasing his E damage early, and reducing the cooldown by 1-2 seconds could help with waveclearing early on. And giving him more base mana will help his sustain again. His W cooldown could also be decreased lategame, to compensate for stacks lost during the time, allowing him to single out targets, and his ultimate could allow him to gain a bit more Hp, armor and magic resist. This could help with the nasus jungle problems in which he stacks quickly due to the large monsters early, and then steamrolls it down late splitpushing, slowing it down a bit, but adding instead more lane pressure, lane sustain, and some different lategame scaling instead of the pure Q damage.
Edit: It might also be a good idea to decrease the ult cooldown by a few seconds early, so he use it to farm as well, maybe a 10-20 second reduction on the level 1 ult.