This won't add a insane amount of skill expression but still might make you think a little about how you use your other basic abilities in what I think is a good way, and cause enemies to slightly think more about how they should interact against Nasus.
Here's the changes (I won't include numbers because that's just a balance thing):
W: Have it slow and cripple for much less.
New Effect: Gain a bonus to both when withered targets are on top of spirit fire.
E: Reduce the tick damage (not initial damage, that stays the same) by a lot. Reduce the armor shred by a lot. Reduce the duration.
New Effect: Tick damage and bonus effects (this counts for W bonuses, and the armor shred from E) get increased up to 100% more at the epicenter of spirit fire.
New Effect: Spirit Fire gains in maximum duration (to a cap) for each tick for every champion that's in it.
Here's an example of how a enemy might use this information to make fights with Nasus slightly more interesting: Lee Sin is dueling Nasus, and Nasus has pressed W and E, immediately after Lee kicks Nasus away and then Q's and follows up to continue the fight out of the spirit fire and not get nearly as crippled as much from W.
The main point of these changes are not to buff/nerf him or make him more oppressive, just shift power around in a more interesting way. Don't comment a change is broken, because I didn't include numbers and those can always be tweaked, I'd prefer discussion of things you could do with his kit, or criticisms of why these changes aren't healthy for Nasus or his enemies in terms of gameplay.
Edit: Some really good suggestions from the comments. If I made changes to this, I'd make it so W goes back to being a flat amount of slow and cripple (not interacting with E, instead going back to live), However would *ground* enemies that are withered while on spirit fire.