This build is suppose to be a fun build you can try in norms, not ranked. Also taking this build requires an already very tanky team. Don't go damage Thresh with only one tank on your team, go tanky Thresh and initiate. If you have 2 tanks or more already, and feel like switching up the build, try this build out. I personally tried this with very satisfying results.

Now you may be wondering where on earth I got the idea to make this build, and it was actually in a game. I went Thresh support and my Twitch ADC afk'ed. So I had to 1v2 bot lane with mediocre results. Then I started farming mad top and bot. Split-pushed and farmed like crazy, while still dropping in on team-fights. By late game, even the enemy fed Vayne was no match for damage Thresh. All I had to do was right-click and instant death. Quite funny reactions XD

Simple! No not really, just stay in lane until you get turret, roam when possible, usual stuff. But once it's around 20-25 min or whenever people start grouping, split-push like mad. Farm and split-push all day and night. The attack speed is ridiculous and after 4 items turrets die in 5 seconds to your hook machine gun. Basically farm like a Vayne. But don't just ditch your team, support your team in team-fights and when required. A good player knows when to split-push and when to group, that applies here. I tried this build out and by late game I had the second most farm on both teams (enemy Vayne won by ~35), had solo taken 4 turrets, and could one-shot enemy Vayne. Litteraly one auto dead. Thresh E passive makes autos do 1000 more damage when fully charged at late game. Plus Stattik, plus Infinity Edge, plus 100 percent crit chance, plus Blade of The Ruined King passive damage, and you can start to see why this build actually works.

Morgana:

Quite annoying due to her black shield, bait it or E first if possible. Avoid her snare as if you get hit, you are not tanky and will take tons of damage, plus it heals her.

Leona:

Be sooooo careful when hooking her, a bad Thresh will pull a Leona right to their ADC, letting Leona easily do her combo. Also her resistances gained by her W are very large and will lower you and your ADC's damage significantly.

Blitzcrank:

As a Blitzcrank main I can tell you, be very careful with how you do your combo. Standard combo is Q,Q,E,R,Auto right? Don't take your Q, Thresh will pull the hook so that it just reaches his E and if you ult, the E pull will just tip the wall, thus it can't be flashed, and you can't exit the combo. So combo is Q,R,E,Auto. You may ask why to not do the first combo. Well there is a small window in between when you take your hook and when you flay on the first combo, that window is just big enough for a Blitzcrank to knock you up and hook your ADC.

Soraka:

Soraka is quite annoying because she has great zoning and poke for a support, plus two heals. You actually want to focus her in bot lane all-ins. If you don't, she will heal her ADC while dealing damage and snaring your team and healing herself with her Q. Only not focus her if the Soraka isn't very good at Soraka.

Nami:

See above, she and Soraka share alot of playstyle. Just bait Nami's bubble out, and trade/all-in while it's on CD.

Taric:

Taric is quite a unique support. He drains mana hard at early levels and by dodging a couple stuns and forcing him to heal, he will literally have no mana and be useless. Just all-in then and you will pick up an easy double for your ADC.