Buff Thresh or rework him.

I get his kit might be have it's strengths, but compared to other champions (cough cough Janna, Lulu, Leona, et al.), he is actually under-balanced. His play style is extremely punishing if he misses his combos and a tad bit risky if he lands them. He is only oppressive against players who do not know how to play around thresh or against players that are just bad, which is rare since his kit is pretty old with everything else changing. Against decent players, he isn't as oppressive as you think and against good players, he seems quite weak. His kit makes him a hit your combos or become useless for about 15 secs. I have no problem with this as it is a clear and fair weakness. However, his moves are target specific and if you do land your combos, you have to make sure your teammates can secure the kill otherwise you will kill yourself, your team and or waste summoner spells to barely escape alive. It's not as simple as landing all your combos to deal damage and have a good trade. Either you must all in with the intent to kill or play the back line and save your abilities to barely disengage. If you do end up trading, his kit leaves him battered from tanking multiple champions before he is able to reach safety.

He's supposed to be a better blitzcrank, but he's not. They're even in terms of usability despite thresh's more diverse kit. At least Blitz pulls his targets through terrain which I'd trade for his lantern any day especially since more times than not, teammates don't even grab it. Thresh has to reactivate his Q to get close to them use his only team saving ability to hope his team follows up. Reactivating his Q is either useless or too risky late game as you will dive into the enemy team that can melt you like butter even with your passive. He's an early game carry but falls off hard mid game and even more late game and becomes absolutely reliant upon his team mid and late game. Hard engage champions like Leona and Nautilus and taric, despite playing Support, can solo kill adcs, mid laners, go toe to toe with junglers and top laners when their team fails to do their job but thresh cannot. Early game, his kit makes his damage seem more overpowered than not since he uses all his cc to secure kills. The only damage-worthy thing about thresh is his flay passive and his ult which is more difficult to land than you think. Since his flay passive takes a long time for it to come back up it is only good early game. If he does decide to do his job and engage, despite landing his combos, he cannot solo kill Anything mid and late game. He is then left to hope his teammates are capable and competent enough to cleanup. If they do not, then he has to waste all his summoner spells and items to survive. If he doesn't, more times than not, he is dead. It takes an extreme amount of skill and experience to effectively play thresh and even more to make him seem unbalanced. In the same token, he isn't rewarded enough for successfully landing all his combos despite having a good bit of weaknesses. Blitz on the other hand, is more rewarding for landing his combos since he can almost one-shot adcs at all ranks and pull the enemy all the way to his team. Thresh has to go all the way to the enemy which is easier for the enemy to get away and even when he lands all his combos, he does less damage than he should be doing.

Before the rune changes, you could build tanky and build damage with combinations of runes and masteries allowing him to not only carry in the early game, but also in the mid game. He still fell off late game. Now, you either build tank and sacrifice damage or build damage and sacrifice tankiness and hope you get ahead in the early game. He went from having high-kill potential to having okay-kill potential. If Thresh doesn't receive more base damage, it would be better for thresh to have a shorter cooldown on his flay active. If you don't want to lower his flay cd then make it to where his flay can flay further for better engage or disengage since fleet footwork, phase rush, and other runes make it easier to stick to champions and run away. If those still don't seem fair, then a rework on the champion would be great. I used to main morgana support and then once I learned about Thresh, I instantly made him my new main. With playing him over and over, I could see where he was strong then and strong now, what his weaknesses are, how play with against different supports, adcs, and team matchups, and the diversity in his kit. Compared to other engaging champions, he isn't where he used to be and with the combination of the tank meta on top of new supports being seen in games and runes, it is harder for Thresh to hold the same weight in games as he used to.