The real problem is how easily, Nasus and other immobile fighter type champions, just get chunked before being able to reach a target. ADC's like Cait and Jinx can effectively kite you solo with just an item or two due to their massive access to CC in the form of roots and slows.
At least Garen can cleanse a slow with his Q which also happens to be hard CC, silence. This also allows him to proc CoC and make his survival even better. Same can be said for Darius. Both champions can access, easily, BC and CoC safety. Neither work for Nasus. Both champions have a form of damage that allows them to do more damage then it seems they should. Darius's Bleed and now Garen's Spin to Shred.
Also most split pushers have some form of decent team fight mechanic. Tryn can deny death for 5 seconds and spin into a team from the jungle and slow them if they run. This allows him to survive the engage, possibly kill one or two champions and slow them down enough for his team to clean up. Xin and Wu are also great DPS split pushers who have access to some decent mobility and aoe ults. Nasus just falls into the unfortunate category of immobile juggernaut split pusher. His job is much harder to split push as he has no reliable mobility to get away or juke and usually must stand and fight. And even this he can barely do b/c so many champions have access to BC or make better use of other pen/lethality items that he can't reasonably build.
So TLDR: Nasus should get some %shred on his aoe and it should burn the target and reduce their armor for as long as they stand on the zone and up to 1.5 seconds after stepping off. I personally like to play Nasus as a team fighter as I think split pushing with him is far too risky as it usually means his death if more than 2 come. Also making IBG melee only and bumping it to proc 125% of base AD would be a slight way to buff Nasus back to a better state as Triforce often takes too long to build and you deny yourself crucial resistances from leaving IBG out.