I would like to see Thresh ADC restored back to his former glory

But what part of his kit could be changed without pushing him towards the OP state?

I find ADC Thresh successful due to a few things:

High Physical damage autos (E) gives you a ton of burst damage (1/2hit-KOs) Inbuilt AP/ARMOR generator from passive Tons of CC which is excellent in lane against champs like Riven (over 90% WR against Riven as Thresh over 20 games)-- Flay can really help punish bruisers who use their escape tool as their engage. Burst. Burst. Burst.

Standard harass = AA, Flay, AA, Thunderlord proc. AA, Flay, AA, Q-E, AA. @ Level 6 after putting them in kill range = R, Q-Walk backwards into wall, E, AA, Thunderlord proc.

I have bursted many people surprisingly with a single AA or the R-Q-E-AA-TLD combo.

The problem with Thresh ADC later on is he becomes a high priority target for teams with smart players. If I choose to CC a target in a teamfight, that leaves me exposed and my damage is irrelevant.

But if I can manage to avoid wards, finish my RFC (rapid firecannon) & one-hit crit build, then I can usually pop 1-2 people off by waiting around the corner or waiting for them to engage on an ally.