Obliterum is going to be the new and big thing in Legion! Obliterum is used to upgrade crafted items by 5 ilvl. You'll need 100 Obliterum Ash to craft 1 Obliterum.
You can only get Obliterum Ash by obliterating crafted items in your Obliterum Forge (pictured). The amount of ash you get per item depends of the material cost of the destroyed item.
The less materials (both different materials and the amount of them), the less ash you'll get. Low cost crafted items can give 1 ash and very costly items requiring rare materials if not epic materials, Alchemy trinkets as an example, will earn you around 100+/- ash.
~~~DEMONSTEEL STIRRUPS~~~ $$ = 1-2k profit
This item increases your mount speed by 20% and lasts 2 hours (8 hours if the user is blacksmith). You can sell these for people who are going to do challenge modes but this won't be much of a gold gain in the first week or so before people start doing challenge modes.
~~~CRAFTED GEAR~~~ $$$? = 10-30k profit
Crafted gear will always be bought at the start of expansions by hardcore raiders or whomever has a lot of gold to spend on gear. Crafted gear will sell highly but it will very costly to make too. Also there is a low chance of prismatic sockets in the items compared to raid drops or titanforged heroic dungeon items.
~~~FEL CORE HOUND MOUNT~~~ $$$$$ = 100-500k+ profit
Blacksmiths can make the fel core hound mount, and it has no profession requirement! This makes it a very good item to craft and sell and if you're one of the first on your realm then you can earn maybe a few hundred thousands from it. NOTE: The recipe for this mount drops from Heroic Tichondrius in the second raid. It has a 100% droprate for BLACKSMITHS and multiple blacksmiths can loot it at kill. It'll therefore take atleast a week or three before someone gets the recipe. You also need a really good progession guild to get a fast heroic kill in the second raid in the first weeks. PSA: It'll take a long time to get the recipe, get the mats and then find a buyer unless there's a rich player wanting the mount ASAP. 500k+ is a shot for the stars but I'd estimate 100k is easily a possible profit.
Rate of possible gold gain: [5/10]
~~~ENCHANTS~~~ $$ = 1-5k profit
Enchants are
expensive as **** and you won't get epic crystals easily. No more rare disenchanting and getting some epic crystals so you gotta disenchant epics. And nobody's going to disenchant their first epics when they're obvious upgrades, and the enchants aren't that much of an upgrade. Hardcore raiders will need them at first, but they most likely got deals with other enchanters or guild enchanters for them to farm for them. The epic crystals might cost 2-3k each the first week and neck enchants are going to cost you like 15k, which won't be worth it for some.
~~~PETS~~~ $$$$ = 10-30k profit
The pets are ridiculously expensive to make and they'll probably run you 20-30k just to make one, but they'll sell for most likely double the cost.
~~~TOMES~~~ $$ = 1-5k profit
These are going to be in high demand and always sell, probably even after legion too.
People will always want old enchant transmogs and these can be farmed by doing old raids and instances and you get the mats easily. Or just buy on AH where it's probably way cheaper.
Tip: Savage bloods! Buy them now or in start of legion where people want to get rid of them. You'll need 15 for Draenor Tome, and they'll increase in price in legion when people start making these.
Rate of possible gold gain: [3/10]
PSA: Engineering is possibly the worst choice for a profession with the intend of making gold in the first week or three of Legion. Engineers CAN make some gold, but it won't be as fast or as lucrative as the other professions.
~~~TOYS~~~ $$ = 5-20k profit
Most of the recipes for engineers in Legion are toys, but they won't be needed for leveling or rushing content. The toys will sell but there won't be a high demand for them unless some of the toys can be exploited for etc. challenge modes, raids, leveling or PvP.
Rate of possible gold gain: [8/10]
Inscription is probably the best profession choice for the start of Legion in terms of making lots of gold fast. I'd highly recommend inscription but you'll need skill level 700 from the start and you're going to need quite some gold to make it really worth your while. It can be done without too much gold too, but not at the same rate.
~~~DARKMOON FAIRE CARDS~~~ $$$$$ = 20-80k+ profit
DMF cards are always going to sell and for a huge amoount in the start of a new expansion. Hardcore raiders are going to buy these instantly when they show up on the auction house and if you're one of the first to craft these extremely expensive cards and/or deck, then you can earn insane amounts of gold.
~~~TALENT TOMES~~~ $$$ = ~50-200g+ profit per tome
These tomes to reset your talents outside of a rested area are already in the pre-patch. I'd suggest making tons of these now and save them for the last days of WoD or first days of Legion. People won't leave leveling to go to rested areas to reset their talents. Selling these in trade chat can earn you a lot of gold for little work.
Rate of possible gold gain: [3/10]
~~~JEWELED LOCKPICK~~~ $ = 30-200g profit per lockpick
Jewelcrafters can make lockpicks now but these won't be needed at release, so there won't be much sales at release. Still a viable opti
~~~CHATTERSTONE~~~ $ = 1-5k profit
It's a pretty useless toy where when you use it, it'll give a yellow or pink text pop up saying "Crystal activated" or "Perfect Crystal Activated" if lucky. They don't have any effect but you'll gain an invisible aura saying this. Since it's useless it'll not sell a lot or for much, but for collectors it'll be another toy to collect.
~~~JEWELCRAFT~~~ $$$ = 10-50k profit
Getting all the rare mats for this toy is going to take a long time. Like maybe a week of farming and prospecting. You can buy the mats though.
The toy itself is ridiculously useless. It's a game of Candy Crush with WoW models. You don't even play it but a grid comes up with miniature models in front of your character like red jewelsaber mount, blue crabs, white MoP alemental etc.
~~~RINGS & NECKLACES~~~ $$$$ = 10-20k profit
Rings and necks at release will be quite useful. They can be upgraded and they are 815 and they got a prismatic socket.
Jewelcrafters can make CLASS SPECIFIC necks that gives a bonus for the class. Some people want these so they can be worth quite a bit.
~~~GEMS~~~ $ = 100-200g profit per gem
Not a lot of prismatic slots at legion release, only in upgraded items and JC crafted items. So you won't sell a lot of these until people start raiding. The first gems made the first days will be worth a lot though. Maybe 1000g each gem.
PROSPECTING TIP:
I'd advise to mass prospect everything if you want to get as many gems as possible though in the start to craft as much as possible.
Later in the expansion (a week or three) begin focusing on prospecting normally, NO MASS PROSPECTING. Gem chips you get have a specific color which indicates what the next gem you create is going to be. This is really crucial for getting the gems you want. Mass prospects can give you chips though and if they do, then it means you need to do a single prospect to change the color.
Rate of possible gold gain: [5/10]
~~~TOYS~~~ $$ = 1-5k profit per toy
Leatherworkers can make 3 new pet toys and these will sell but not for much and they don't have an urgency except for pet battle fanatics. There's a pet leash, flaming hoop and a pet bed. You can also make a new toy chair which can be sold for maybe the same price as the other toys. Leatherworkers can also make some items that makes you inable to be dazed in Broken Isles for 2 hours, but these most likely won't be in any demand and won't likely work in challenge modes. If they do work in challenge modes, then they would be worth something like the blacksmith demonsteel stirrups.
~~~CHEAP CRAFTED ITEMS~~~ $$ = Obliterum Ashes
Leatherworkers can craft pretty damn cheap rare items that can be obliterated for a decent amount of Obliterum Ash. Capes and other items cost roughly 15 scales (skinning material) and with proficieny it'll cost 12 scales. These items will give you 3-5 ash. So they are a pretty cheap way to obliterum ash that will sell for maybe 10-20k each at first.
Rate of possible gold gain: [2/10]
PSA: I'd advice against going Tailoring if you want to earn a lot of gold in Legion release.
~~~BAGS~~~ $ = 100-300g profit per bag
The bags cost nothing to make so won't sell for much. Also they are only 24 slots. Embersilk bags are 22 slots and sells for like 300g and less in legion.
I'd recommend making Hexweave Bags and sell those instead in Legion.
~~~RUNIC CATGUT & SHARP SPRITEHORN~~~ $ = 1-10g per item
These two important reagents will become available after a tailoring quest, but not all tailors will do the quest and wonder where to get it. A lot of people will be lazy and just buy them off the AH. So you can buy a lot from the AH and mark them up on the AH, but it'll probably be flooded really quick like the old talent reset dust you could buy for 10s and sell for 1g on AH.
~~~RAID COSMETIC SETS~~~ $$ = 1-2k profit per set
These recipes drop off legacy raid end bosses like Ragnaros. When created it'll make an item you can place on the ground, and when your team/raid press it, they'll get dressed in a tier cloth set like T1 warlock, mage and priest sets.