Renekton is underpowered if anything. Tons of champs with different styles can go close enough to even with him early game and either outscale him damage wise or just become unkillable compared to him. Renekton doesn't become truly tanky. He can get semi-tanky at the cost of having no damage late game - black cleaver alone even with full fervor stacks does meaningless damage late game. He can go 2-3 damage items and have respectable late game damage, but be way too squishy. His cd's are longer than other spellcaster bruisers like riven or he would probably scale better. He can be abused by forcing extended trades rather than letting him ff while his spells are on cd.
Grag, dar, riven, kled, shen, rumble, etc. can all fare perfectly fine vs him as long as they don't go too ham and give him a huge opening. Renekton is actually fairly weak until he gets all 3 basic spells. Grag wins trades until renekton completes black cleaver if he plays it smart. He can interrupt renekton's slice-n-dice with his bodyslam, get a shield, percentage hp damage via drunken rage aa, and barrel off while renekton gets auto w auto q. Other ap bruisers/tanks like galio had/have similar combos.
If anything, renekton should have either better scaling with damage (like same ratios but scaling off total ad instead of bonus ad) or gain some resistances when his ult is up. Because he scales way worse than every other champ with both base stats and with items. On top of that, his cd's are longer. And a smart opponent, as I have already said, can play safe and deny him real kill pressure 1v1. If renekton doesn't get kills in laning phase and start snowballing, many champs can easily overtake him by the 2 item mark. You shouldn't need a jungler to have kill pressure on someone who is so early-game focused as renekton considering how hard you fall off. I'm not saying you can never get solo kills - but they are remarkably harder to get 1v1 for renekton in the early game than they are for say a jax or fiora late game. If those champs get such easy wins 1v1 late game because they are hypercarry toplane duelists, then it would be logical that renekton have an equally easy time achieving that in the early game - being an early game "lane bully" duelist. But he doesn't have it half as easy to pick up 1v1 kills - this holds more true the higher you climb.
On the flip side though, if you give renekton 2 kills in lane, he is probably the scariest champ in the game early game. This power level is more or less what it is like for a fiora or jax late game 1v1 regardless of if they have a kill lead or not. Renekton achieves with kills early game what other champs achieve late game with just items.
So, renekton isn't what I would call balanced, but I doubt they will do anything to him in the near future. I could see shaving a small bit of power off his early game when he gets ahead - cuz he does become super oppressive; however, if they do that, they must also buff his scaling to be more comparable to say a darius or even a riven. They could do that, but I feel that most attempts to do so would be a painful process and end up making him lose his lane bully identity.
