I consider myself a purist in that I would prefer classic to be released as close as possible to the original. Where that is on the spectrum of patch content will be up to blizz. I'm also not vane enough to believe that any single state of the game is the "Ideal" location we should be aiming for. There is common ground that we can agree on, and I would welcome more than a few changes as long as they are made in the spirit of keeping classic on the vanilla rails.
Leatherworking has three specializations: Elemental, Tribal and Dragonscale. I’d pick up Tribal one. It allows to craft Devilsaur set items. Hunters, Rogues, Warriors, Feral Druids, even Enhancement Shamans and Retribution Paladins are among your potential clients. On any realm there is Un’Goro Mafia on Horde and Alliance, they control whole Un’Goro Crater zone and farm Devilsaurs 24/7. They are making thousands of gold. With Dragonscale Spec you are able to craft Black Dragon Mail set. Good demand as well among DPS classes Warriors, Hunters, Shamans, Paladins, for those who wants Bis Pre-Raid items fast and easy.
I'm not all that hardcore on "Classic", I want to relive the old zones and the old world and it seems like most people I'm taking to is feeling the same. We would prefer to have quality of life improvements regardless of them actually being in the game at in 1.12.2 or not. Like the "Automatic Quest Tracking" to actually track it properly without you having to actually trigger progress in the quest for it to show up would make perfect sense. It's no real need to make such a thing behave nonsensical just because it's "authentic".
Where to get materials and reagents you may ask? Buy for cheap off farmers in bulk or track Auction House lots. Get in touch with most big sellers. Communication is important here, send a whisper or write them a letter. It would be good for both of you to establish business relationships. Constant suppliers and buyers is convenient for both parties to save the time and effort.