Here's the $10.00 version of my $0.02 cents:
If your goal is to just make gold, picking 2 of the 3 gathering professions tend to be your best bet. Mining and Herbalism tend to be the more lucrative of the 3 gathering professions because of how many production professions use their products, but if you use Gathermate 2 or a similar addon, it can clutter your map very quickly if you’re tracking both at the same time. Skinning is a nice alternative if you want to cut down on map and mini map clutter, but it tends to be less lucrative because of how easy it is to obtain skins combined with skinning products not being as much in demand as Ores and Herbs (though Legion helped that a little bit by making recipes in professions outside of Leatherworking use some skinning products). Still, it can be lucrative as well, it just requires more time and quantities of skinning products to get there. A little hint: Falcosaur Swarm WQ's are a gold mine for skinners in Legion. Finally, you can take Enchanting and/or Tailoring to use as gathering professions if all you want to do is get more cloth / enchanting mats to dump on the AH, though some might consider that a waste, but who cares?
If your goal is to stock up on mats for the professions of your alts, than you should pick gathering professions that will support those professions. For example, if your alts have Blacksmithing, Jewel Craft, and/or Engineering, picking Mining for one profession will help that. If you have Alchemy or Inscription, then Herbalism will help those alts. I wouldn’t recommend taking Enchanting or Tailoring if you just want to get enchanting mats or cloth for other alts that have that profession or use those mats. You would be better off just doing the work on your primary at that point, though there’s nothing against having multiple alts with the same professions if you want to.
If your goal is to be mostly self-sufficient for one or two toons, you should pair up a gathering profession with another profession that utilizes those mats (see above).
If your goal is having extra utility or quality of life improvements, Engineering is the way to go. Engineering offers you some nice tinkers that can help you escape sticky situations or to get out of a danger zone real quick if you’re slow to react to something. I can’t tell you how many times a Goblin Glider or Nitro Boosts has helped me escape from a fight that went south for one reason or another, not to mention all the ways it helps cuts down time on things (those caves suck for mounts). However, that said, Tinkers can occasionally fail with painful consequences, and I know the Nitro Boosts share a cool down with potions. Finally, Engineering can also let you craft your own guns and scopes if you choose to be BM or MM.
If your goal is to make products to sell on the AH, any of the production professions can help you achieve that. Generally speaking, the more profitable ones are:
- Enchanting: Item enchantments are always in demand, especially from all the raiders out there. And every time a new raid comes out, people need replacement enchants for all those upgrades they’re getting.
- Jewel Craft: Like Enchanting, gems are always in demand for the same reasons. However, at least on my server, Jewel Craft has a lot of competition, and getting epic quality gems without having to buy them can involve more work than getting most Enchanting mats.
- Alchemy: Like the other two above, potions and flasks are always in demand regardless of whatever raid tier is currently active. Also, Alchemists can transmute various mats from one form to another, and can make a *BLEEP* ton on that too depending on the economy of your server.
Of course, this can vary from server to server, so it’s always best to do research and try to fill a niche that’s lacking on whatever server you play on. And if you’re not attached to a particular server for some reason, higher pop servers tend to have more players buying and selling, but also increased competition. You don’t have to worry about the population of each faction any more as far as being a trade baron / trade baroness goes, they made AH’s cross faction back in WoD. Meaning, either faction can buy your stuff instead of just the players on your faction.
Finally, if your goal is to supply your own toons with gear and you’re a massive altoholic like I am, just learn one of everything and use your mains to do the gathering. Just be sure to establish a private guild if you can, resources and mats take up A LOT of space, and having at least one toon act as your “bank toon” that has their own private guild will help with storage space IMMENSELY.
* Bonus Note: If your goal is to make gold, consider taking up Fishing as well. Fishing is a secondary skill any character can have in addition to their 2 professions, and is a laid back way of making cash when you want to just sit back, relax, and not be running your rear end off on a constant basis. And right now, it’s incredibly easy to level compared to how it’s been in the past. Just join fisher friend groups in the group finder, then join them in fishing up each fisher friend’s currency. As you do, you’ll catch special, zone specific bait that can be used to catch rare fish that will give you +5 Fishing for each one you catch, then throw back. It’ll also help you get the legendary “Underlight Angler” fishing pole that much quicker as well, and most of the fish you catch sell for between 4 gold and 12 gold on average, and just wait till you see how awesome fishing up “sleeping murlocs” is for your bank account and leveling the Underlight Angler
Just be sure to do it much sooner rather than later, these Legion Fishing groups won’t be as plentiful once BfA comes out.
Anyways, I hope this helps you out. Oh, and one last thing, buy an “Ultimate Gnomish Army Knife” regardless of what you decide to pick, it has every tool you need for every profession with the exception of a philosopher's stone that's need in Alchemy.