Make Thresh both melee and ranged.
Thresh is a neat guy and probably one of the best designed champions in league he combines utility, play making, and mechanics into one fun guy to play that isn't super binary or unfun to play against. He also has this theme of a melee/ranged hybrid from his auto attacks to the fact that every ability in his kit can be used to either create of reduce distance between and other champions. Due to his kit thresh can't ever really step out of the support role which is fine in ranked and competitive, you should always play and build a champion in the most optimal way possible when there are stakes involved. But in normal fun games sometimes i want to bring thresh top which leads me to my proposed buff; Thresh's basic attacks are counted as both melee and ranged and will choose the better of the two when calculating on hit effects and item passives. It would probably be tacked on as an additional innate passive. This really wouldn't affect a standard competitive support thresh build but it would definitely help help those brave summoners who want to help thresh break out from his imposed position.
There is are only 10 items in the game that have different effects based on whether or not a champion is range or melee (
) and none of them exist in a standard support build. The only item i could see being pick up for a support thresh is
but i highly doubt it would usurp
and even if it did a 1 second 50% slow would hardly have the potential to break support thresh. I think opening the hydra options to thresh would make his solo lane viable for fun normal matches but it would by no means break Thresh. It would add to the cool Melee/ranged hybrid fantasy that thresh tries to deliver by making it exist as actual gameplay.
P.S. Also sort of insane power farmer jungle sated Thresh doubling the damage from his passive would be hilarious