I want to compare Cho'Gath to Zac.
Yes Zac has the better passive allowing Zac to revive up to half health and giving 4% (of his health) heals, but Cho'Gath's isn't far behind as it gives both mana and health for any kill.
Compare the Q's.
Cho'Gath sends out the damage 80/135/190/245/305 (+100% AP). This ability has a area effect but can be avoided. If not avoided, enemies are sent into the air and afterwards slowed. Cho'Gath sends out this ability from a safe distance. This ability has a low cost for late but high for early.
Zac sends out damage equal to 70/110/150/190/230 (+50% AP). The ability has area effect but can be avoided. If not avoided, it deals damage and enemies are slowed. Zac sends out this ability up close in a 550 range. This ability has a 4% health damage to Zac.
Zac's is comparably weaker, dealing less base damage and lacking in AP. The price for using Zac's Q does not diminish in late game, and this carries a high burden.
Compare the W's.
Cho'Gath sends out a feral scream that deals 75/125/175/225/275 (+70% AP) damage. This ability is also an area effect, that deals to an range of 700 (there's a typo on the Champion Info page btw). Those hit are silenced for a short period. The cost for this goes to a high amount in late game.
Zac sends out a area damage of 40/55/70/85/100 (+4/5/6/7/8% (+2% per 100AP)) to enemies in the radius. The area effect is at a range of 350. If Zac collects blobs, the short cool down is lessened by a second per blob. The cost is 4% of Zac's health.
Though Zac's W can deal some damage, Cho'Gath deals way more and even has a silence.
Compare the E's.
Cho'Gath is a toggle-able passive that allows Cho'Gath to deal 20/35/50/65/80 (+30 AP) bonus damage. This ability is also an area effect, dealing damage to those behind the one he attacks. Those hit are dealt the bonus damage. This ability costs no mana, and has a range of 500.
Zac flings himself to an enemy dealing 80/130/180/230/280 (+70% AP). This ability also has an area effect, but is can be avoided. Those hit are knocked back for 1 second. The range for this ability is 1150/1250/1350/1450/1550 though the time to get ready increases from 0.9/1/1.1/1.2/1.3 seconds. The cost is 4% of Zac's health.
I would want to say Zac wins this one, but since Cho'Gath's if free, and deals damage to multiple enemies faster Cho'Gath's E is better since Zac has to channel to send out his powerful attack.
Compare the R's.
Cho'Gath deals 300/475/650 (+70% AP) true damage to his opponent (deals1000 (+70% AP) damage to minions). This ability has no channels needed and is an automatic damage. If enemy is killed, Cho'Gath gains 90/120/150 health per each time he kills the enemy (stacks up to 6 times). Cho'Gath also gains 3.8/6.1/8.3 bonus attack range per stack. The cost is 100 mana. Cool down is 80 seconds.
Zac deals 140/210/280 (+40% AP) damage when his ultimate first hits an enemy, the damage is reduced by half if it hits the same enemy. Enemies hit for the first time are knocked back for 1 second, they are not knocked back for the next few. Zac bounces 4 times, each dealing damage. (The max damage is 350/525/700 (+100% AP) if the enemy is hit all 4 times.) There is no cost for this ultimate. The cool down is 130/115/100 seconds.
And again, the winner is Cho'Gath. Why? Because he deals the damage in an instant, and get the kill and gain the buff. Zac on the other hand may deal more damage in the end, knocks the enemy away from him, Zac then may only be able to deal 3 out of the 4 bounces on the enemy dealing less than Cho'Gath's ultimate in the end.
Zac is too weak. He could really use some buffs to his sustain, or at least his damage.
We could do 1 of these two buffs:
[P]
Zac is spread out into 4 pieces of himself that each have 14% of his current health. If they all reach the center, he revives with up to 50% of his health.
Zac is spread out into 4 pieces of himself that each have 25% of his current health. If they all reach the center, he revives 100% of his health.
[Q]
New Zac can now collect blobs.
[W]
Cost lowered to 3% from 4% of Zac's health.
or
[W]
2% per 100 ability power decreased to 2% per 50 ability power.
[E]
0.9/1/1.1/1.2/1.3 seconds decreased to 0.7/0.8/0.9/1/1.1 seconds.
[R]
140/210/280 magic damage increased to 140/220/300 magic damage.
The first one allows Zac to be more tanky and better in his late game. The revive is now less easy to one shot, you guys know its true, and since he relies on health, 50% is too harsh. The other two are for better sustain as well.
The second is for his damage. Tanks in this meta are allowed to deal good damage, his W should be no exception. If were able to get 600 AP, why do I only get +3% damage. Now you can deal +6% damage if you went from tank to AP in late game. His E already deals good damage, but that time to activate is something else. I can handle waiting 1.1 seconds on max. His ultimate also should deal more if every other bounce deals half the damage. Since his AP can go up to 100%, allowing him to deal up to 350/550/750 damage isn't much if he lands every hit.