TL;DR Nasus needs base stat improvements, itemization improvements, and/or greater levels of tenacity from how many stacks he has.
Nasus' biggest issue is that the reward for playing safe and stacking well has been marginalized over the last year for several reasons.
- Mobility Creep - which was becoming a huge problem in league and riot has been gradually trying to address it since preseason
- Slight nerfs to several items he builds. This has occurred mostly over the last year since the juggernaut rework. The items include
. With items such as 
all being buffed. Over time, the slight nerfs to the several items has netted a loss on Nasus relative to other champions. This is not saying that the buffs to other items have not mitigated some of the damage to Nasus, the overall net effect has still weakened him.
- The amount of CC in the game or the trend of high CC related comps is problematic for Nasus. At the moment there is not enough tenacity items for Nasus to buy which has made Nasus less helpful in team fights. Champions with higher levels of CC have been or were in the past prominent top laners:
. in particular has still stayed in the top lane, but he is a champion that can loose lane to a Nasus and later still keep him locked down in team fights because of the high utility in his kit. Just one Ekko can really keep Nasus from gap closing.
- Nasus received a soft nerf with the mastery changes - with most of the masteries being relatively bad on him to OK at bet. Grasp of the Undying is decent on Nasus, but when this keystone was strong EVERYONE in the top lane was taking it. This marginalized his sustain in lane relative to other champions, making for fewer backs for opponents. It also made it so that Nasus could not simply put all his points into early survival. The tank/resolve tree is much worse than it use to be, with very few providing "tanky" stats. The effect on Nasus is the his base stats received a nerf with Nasus players having to rearrange their rune pages to provide the needed resistances and health needed to get through laning phase. I had to completely rearrange my rune pages on Nasus after the masteries were changed just to get comparable stats.
- The pace of the game is faster. The game has been speeding up a bit and it is a little faster than it was in years past. Though they have made towers stronger and lowered death times relatively recently, the trend in armor penetration builds has made laning phase shorter.
Lets discuss the average Nasus encounter with a capable ADC. For this example, lets assume both players are seasoned players with their champions (Approximately Gold). This is an encounter around the 30 minute mark with an ADC. Nasus approaches, ADC kites and inflicts damage but will get withered, ADC kites back applies whatever slow or CC they can use (if they have it). Nasus gets one Q in, but then must wither again in order to kill. Second wither, ADC kites back uses QSS, applies damage and/or CC. Nasus is taking a lot of damage at this point, but not enough to be killed. Nasus will need to apply 3rd wither to ADC. ADC then uses flash and maybe heal to escape over a wall. Nasus gets no kill and ADC escapes with summoner's down.
This scenario omits the involvement of a support or other ally with any type of CC to protect the ADC. True Nasus survives the engagement with an ADC, and maybe can take an objective off it, but he his not rewarded with stacking well or his itemization. One of the reasons that Nasus is much better in the mid game is that enemies have not yet itemized against Nasus. They don't know how strong he is, nor will they coordinate their efforts to chain CC him. The next encounter with him they will. This means Nasus is either a split-push distraction that 2 enemies will have to address together, or he can group get chain CC'd and HOPE that his allies stay with him and inflict enough damage before he dies. This is why is such a great item in the late game for Nasus. That second life in a fight is crucial for Nasus to stay relevant.
Solution? I have a few suggestions that will affect Nasus specifically:
- Improve his base state, more specifically his health or scaling health.
- Grant some type of tenacity passive for his stack count. Something like 1% for every 100 stacks. This will make it so that Nasus is a bit of a force to be reckoned with if he stacks well.
They could also improve some items, but I think that itemization changes could have greater implications than helping only Nasus.
Nasus is largely fine and significant changes to his kit will only alienate existing Nasus mains. I think 
is a champion that takes a lot of patience to play, but the pay off for patiently farming for most of the game is not large enough or guaranteed. This has turned Nasus into a champion that is largely about making decisions. He needs to decide when he should group, when to take objectives, when to avoid grouping, when to exert map pressure, when to farm, and when to chase or zone. Other champions have to make the same decisions as well, but with Nasus' play style largely being geared towards split-pushing, it has made it so that his win/loss ratio is completely contingent on that these decisions. Not based on getting items, stack count, and the mid game power spike.
