Yeah Quinn would be a definite buy for me if I didn't hate bot lane so much, though because of her bruiser-esque nature with her ultimate, and ability to duel very well she could potentially be a ranged top laner/mid and still do well (need to build tanky to utilize ult in a real fight).
As for ADC's, Corki is a very strong pick along with Graves and Ezreal, very spam heavy champs with lots of nuke potential even if their steroids aren't as powerful as some others. If you can take the time to really understand and practice 1 champion though (I get bored of them too easily) I would suggest Vayne as if you can get her mechanical play down well, she's the strongest ADC in the game in terms of damage output and mobility.
And Thresh can work as an on-hit, just not with any AD. The AD scaling portion of his Q's passive is time-based, but he will always hit for a flat amount based on his souls harvested, meaning if you have 300 souls you'll still hit for 300 magic damage every hit. Combine that with a Wit's End and you've got decent damage output, only issues being his attack animation is kind of weird for that and he needs to build quite tanky to be allowed to AA.
Also finally got into plat after 6 wins in a row