Nasus needs change
1st off, I want to make it clear that in no way am I saying Nasus is OP, requires rework, is broken or needs to be nerfed, I'm simply stating his kit isn't "balanced."
Nasus' kit involves a passive 20% lifesteal, giving him automatic sustain. His Spirit Fire temporarily gives 40 armor reduction which can reduce armor value to negative, granting him amplified damage that's on par with a critical strike. Not only does his Siphoning Strike have 100% attack damage scaling, but with adequate farming a single strike rivals the burst damage of most assassins. Then we get to his ult which gives him a bonus 50 attack range and 600 health. Wither allows him to reduce his opponents movement speed by 95% and attack speed by roughly 50%.
With what's been mentioned thus far, Nasus has lifesteal, flat armor reduction, a slow, a heal and bonus Siphoning Strike attack damage without assistance from runes, masteries or items.
His magic damage ult can deal 3600 magic damage against those with large health pools or anywhere from 60% to 90% of max health magic damage. His Spirit fire can potentially deal 430 damage. Spirit Fire has 60% AP scaling.
Not only can Nasus go full tank and still put ridiculous damage causing you to need both MR and armor to not be burst killed if you're mêlée. Regardless of build, his Siphoning Strike can instagib any squishy.
The kit is too versatile, way too many ways to build Nasus and be a threat. From full tank to bruiser, AP tank, hybrid damage max health, he's always a threat.
My fixing ideas would be either nerfing his passive lifesteal at levels 1/6/11 to 3%/5%/7% or changing Siphoning Strike to magic damage.
tormented soil and 
null zone are inferior to it in all aspects. Plus, it also gives the 40 armor reduction, meaning not only do you take the magic damage, but all physical damage (even from minions) will hurt that much more.'
or 
at early levels actually gives you more sustain in lane.
your general goal is to stop him from farming and stacking. His play-style would be balanced in Dota 2 where true farm denial is an actual mechanic. In league however denial is only a timer your trying to outpace. Now his weak early game USED to allow for better opportunity to keep the dog famished, until they put 
this fucking thing into the game! Flask can be bought with 4 pots as long as your willing to reach lane a few spare seconds late. This allows Nasus to sustain against harass with absurd efficiency. Q heals Nasus, charge 120 HP provide 60 mana (X3 Q's per charge) to a champion who has no intention of trading resources with you. So while you expend your resources (health and mana) trying to harass Nasus, His game plan is to stock his resources while ignoring you until he's ready to decide when to deal with you. His kit and start items combined, heavily punish opponents for resource trading or extending into the lane and make sure that they're generally always going to be behind in a 1 v 1 scenario where farming cannot be their first priority. Considering the synergy of 
there are many cases where he won't even need to touch a 
and can save them entirely for replenishing health after a gank attempt. This in turn makes for a very objectively difficult lane. Do You A. harass Nasus wasting resources and missing farm in an attempt to slow him down? Or B. Ignore Nasus and try to keep up in farm so you can remain relevant in the late game? Both are bad options, both can hurt your hurt immensely in differing scenario's. The whole of the affair feels like an non-interactive nightmare scenario of damned if you do and damned if you don't.
For example, one of my favorites is also complete bullshit with a Flask as he turns a rather small amount of health into a heft chunk of early game true damage. He'll literally beat you to death without wasting any mana and juggle his axes ontop your dead pulverized corpse. Then to add insult to injury he'll pop his W take a sip from the flask and be back at full health and mana before you even return. He's got a bit more counter-play than Nasus obviously its another strong case of flask bull-shittery and not even lose to the only example.
and bullied him out of lane with Spirit Fire. Walk up to low health minion without proccing your Q, Teemo uses his blind, use then Spirit Fire + Wither then Q when blind wears off and you win the trade every time.
and farming the backside of his minion wave starting at level 2, which is denying him experience, CS and gold, but gives me very horrible positioning against a gank, forcing me to spend more gold than I usually would on wards.