If you're a new player and buying gold (for some startup money) isn't an option, there's nothing wrong with gathering (especially if you're getting BfA...it'll be good gold initially if you farm BfA mats and sell high). You can make pretty good money just flipping stuff...and because Legion items are super risky now, it's a good time to get spirits of harmony (just buy what you can and sell it in in bigger stacks at higher costs or turn it into ore/bars or lotus to sell when that is profitable). I'd avoid putting all of your gold there though. Sometimes a bot or just someone returning to the game will dump 10+ stacks of 200 spirits cheap and tanks the markets for a bit. Flipping (if you get good at it) will make you more gold in than long run than farming but if you don't have much gold either is a step forward from here. They key to making gold is having the flexibility to lock up most of your gold in items and wait patiently to sell them at a good price but you gotta start small (or cheat like me...when I came back in December I just spent $100 on tokens and haven't spent money on anything other than the Collector's edition since. It's a lot easier to make gold if you already have some.
2. Island Expeditions. What?! you may ask, but it is simple. The transmog that drops, now rather frequently, from the expeditions sells well and for high amounts. Despite its somewhat common drop rate the fact that the loot table is so large means that people seeking certain transmog will be more likely to buy it than farm it. The parroted meme on r/wow that it is unrewarding only makes it better for someone like me. In the past two days from doing Island Expeditions on the side (about 20 total) I got 6 transmog items of TSM values between 15k and 150k. I sold 3 so far totaling 140k (Plundered Squid Caster: 50k because I wanted to be rid of it, Stinkrot Smasher: 69k, Razorfin Breastplate: 28k). Pet drops are a different story and while you can sell them, the fact that they need to be looted for the achievement means that the potential buyer pool is smaller.
But every new character made that wants to craft old expansion stuff will have to level using expansion specific materials. Stocking up on profession materials that are already high in demand can be a great idea. This will primarily be materials used to craft the various mounts. Ghost Iron, Trillium, Living Steel, Pyrium etc. are all great candidates to stock up on.
A variety of people use professions in this game for different purposes. Some just use them for their own needs and some use them because their game is making gold on the auction house. This is part of the problem since blizzard never seems to be able to please both groups lately because they don't spend enough money on that aspect of the game to offer enough of a variety of patterns/recipes.
This top-end difficulty was missing entirely from Legion’s first raid, Emerald Nightmare, which was cleared by top guilds in around 11 hours. Meanwhile the first guild to clear Uldir’s hardest difficulty, Mythic, spent somewhere around 70 hours attempting to kill the raid’s eight bosses. Even after all that time, the guild, Method, still had members say they loved the raid, and that it was one of their favorite in recent memory.
Allied races aren’t much better. You can only earn them by grinding out reputation. It doesn’t take that long but casual players will need to invest a couple weeks primarily to it, and focusing on that grind will take you away from others, like earning gear. Each race has its own grind, but the things you do to work through it remain the same, adding to your boredom.