Thresh needs to be toned down.
I have played Thresh quite a few times, especially in season 4 where I'd almost always get a KDA of something along the lines of 6/0/30. The games would almost be over before the 20 minute mark because of the insane advantage this champion can give to his AD carry.
The average league player is gold, thresh takes about a champion understanding of silver 4 to be good at. He has an absolutely overloaded kit, and if you know how to play him on an even higher level, gold-platinum, the results are stupid. Zyra hotfix stupid.
So let's see what he has at his disposal.
Thresh has a hook(Q) that STUNS an enemy for 1.5 seconds after a 0.5 windup, this also deals magic damage, and the skill's cooldown is refunded by 3 seconds if it hits a target, the lowest cooldown on the skill without hitting something and NO CDR is 12 seconds. Assuming you have max CDR and hit your target, your cooldown is now 5.4 seconds. Keep in mind that the target is stunned for 1.5 seconds, making that into a 3.9 second cooldown if you milk the ability to the full stun.
That alone is a pretty powerful spell to have on a 3.9-9 second cooldown.
Thresh has a W that is a free escape, a shield, a free way to pick up souls from afar and a free bush check. The max return range your ally can travel is 1500. It can also be targeted by ally dashes. Such as Lee Sin's W.
Then we have flay... oh boy. A auto attack buff that scales up to 200% of your AD + all of your souls you've collected. Not only that but it knocks the enemy up, triggering knock up effects and letting somebody like a certain lone wanderer
to proc his ult off of it. Then AFTER all that, it slows the enemy by 20/25/30/35/40 while dealing magic damage. (Note: The auto attack is AD, the active is magic damage just like the hook. This thing is on a 9 second cooldown from the first rank, 5.4 if you have 40% CDR.
Then finally we have ZA BOXO. Thresh summons 5 walls in the shape of a box, each wall lasts 5 seconds and they slow for 99%, dealing 250 magic damage + 100% AP at the first rank alone. Now while the damage can only be applied to a target once, the slow can be applied 5 times, 4 of them lasting for 1 second. So anybody with a big hitbox is almost always going to hit 2 walls at least.
Keep in mind, thresh's passive gives him 0.75 AP and armor per soul, so add that AP ratio onto your magic damage skills.

You absolutely cannot get away from thresh without killing him or having an insane dash/escape ability such as
Flash is entirely useless when facing a thresh that has a brain, as his hook outranges your flash and he can simply flash himself, use his abilities to catch up or an item. If you don't have a gap closer or flash, you won't be leaving where you're standing unless you kill him, otherwise face constantly being hooked(stunned) flayed(Knocked up and slowed) as well going through a 220 shield for him and an ally.
He's allowed to build full tank and zero damage items while still doing a lot of mixed damage and forcing the item to either attack him or never get away, turning him into a lesser version of rammus and his taunt.
Now I love Thresh, don't get me wrong. I even have a beautiful(Uh, pretty bad) drawing from my 3 year old cousin of him on my wall... I love it dearly. He's one of my favorite champions gameplay wise and theme/lore wise. However his current state isn't healthy at all. It is a legit cancer of League of Legends, and not one of those cancer champions people complain about for two weeks then forget, he's always been here and always will be over-powered until he's toned down.
I can't just scream THRESH IS OP ZOMG NERF PLS!111! without putting in my two cents of how he could be toned down, power shifted or nerfed.
Increase the Q windup time from 0.5 to 0.75, this gives the target a little more time to go "OH S#*!" if he isn't skilled or successfully plan to dodge it if he knows how to play properly. (Sidenote, you could increase the time to 1 second for the windup but increase the missile speed by 200. A large time to see it coming, but if you don't take action you'll get hit for sure.)
Souls should give 0.25 AP and 0.50 armor instead of 0.75 for both. This is actually a pretty big one, 10-25 armor in a game can make a huge difference, and if he's getting this stuff for free with no item slot, it can't be that powerful.
Thresh's latern should be able to be brokenshattered by an enemy attacking it 3 times for ranged, 2 times for melee. Enemies get to use bard's magical journey, I don't see why they shouldn't be able to stop thresh and his ally from denying the enemy a kill, even if they set up perfectly for it.
Now of course I'm not saying do all these at once, but take and pick what you'd please.
If you have any questions, concerns or upvotes, feel free to throw them my way. I'd love a debate too.