Most people that have played with or against Thresh can agree that there is something lacking, or just not quite right. Here are my proposed fixes:
The Problem: The windup for Thresh's Q is an obvious tell and a cast delay that makes hitting anything with it almost complete luck.
The Fix: Get rid of the windup.
The Problem: Thresh's ult is on a very long cooldown, does very little damage, and has no hard cc.
The Fix: Make it so the damage and slow from breaking a well is a small aoe, enemies are effected by more than just one wall, and all walls do the same amount of damage. This will make his ult an actual threat, instead of just a, "oh, Thresh ulted? Damn, didn't even notice."
The Problem: Champion souls are worth the same as cannon minion souls.
The Fix: Make Champion souls red so they are more noticeable, and make them worth three stacks rather than just one.
If these changes are implemented, I think Thresh will be up to speed with the other supports in the game.