Dark Harvest AP Bruiser Thresh (ARAM ONLY)
https://i.imgur.com/SErtbho.png
Start with
.
Your build path will look like this most games.

Last item is contextual, either
or
(
can sometimes be fit in but it usually is a hard choice)
If you sacrifice abyssal you will have mana problems and a rough early game, and the 3 AP items are very necessary for scaling purposes. all those items are important and best-in-slot imo so the last slot is really the only one you can be creative with.
Focus heavily on soul collection.
You will be quite durable against all forms of damage, have extremely surprising burst and sustained damage (with potentially high self-healing from ravenous and gunblade), along with all the utility benefits of thresh's kit as well as maxed CDR.
And most deliciously, you will be slurping both red and blue souls all game.
If you make it to late game with this build you will almost certainly be the strongest champion statistically, on both teams, and you will only continue to scale from there. It's hilarious how often ADCs think they can duel this abomination. After all, his armor, AP, and on-hit damage all scale directly with souls. This just compounds the bonuses of his very loaded kit while keeping him tanky.
Have fun