Gathering professions became largely irrelevant in Warlords of Draenor since you could obtain crafting materials from your mine, herb garden and barn.

In Legion, gathering professions are relevant once again, forcing you to get out into the world. So, if you were like me, and gave up a gathering profession in favor of a new crafting profession, should you drop the crafting profession and re-acquire the gathering profession or should you try leveling a new character? There's still plenty of time before Legion hits to do either. If you still have a level 100 character boost available, I would recommend leveling a new character to level 60, and pick up two professions prior to applying the boost. This will boost the two professions and first aid to the maximum level of 700.

Consider carefully your choice of professions if you go this route. It makes the most sense in Legion to have a crafting profession supported by a complementary gathering profession. For example, a blacksmith would be wise to have mining as a gathering profession (sample blacksmithing recipe: Demonsteel Boots). The reason for this is, one of the main crafting ingredients across most professions in Legion, Blood of Sargeras, is currently BoP in the Legion Beta and, as recently as June 10th, one of the developers has confirmed that it will remain that way. The Blood of Sargeras can be obtained from herbalism, mining, skinning and fishing. There's some talk that it can also be obtained from world quests and dungeons. Because it's BoP, there are real problems with having two gatering professions or two crafting professions rather than one of each. If you have two gathering professions, about all you can do with the Bloods is vendor them. If you have two crafting professions, it may be that the only way you can obtain the Bloods is through world quests or from fishing.

So should you rearrange all of your characters' professions to have one crafting and one gathering profession? I sure hope that won't be necessary. It would be nice if they would simply change the gathering professions so that they were all secondary professions that anyone could learn and level. The Legion testing is probably too far along to make such a change, however. Or they could simply make the Bloods BoA or BoE.

Until more is known about this, one thing you can consider doing now is leveling your fishing, especially if you have two crafting professions. To get Bloods from fishing you will need the fishing artifact, Underlight Angler, which requires a fishing skill level of 800 among other things.

There was a weekly Developer's Q&A with Paul Kubit on professions.

Basically nothing got resolved. The BoS will remain BoP, dual-crafters will still be at a disadvantage compared to a crafter/gatherer since dual-crafting will often require twice as many BoS as a single crafter.

Note that while it only takes 2-3 Blood of Sargeras to craft a single piece of level 815 gear, there won't be a re-roll mechanism if that gear doesn't have the stats that you like. Instead you will need to craft the gear over and over again. The gear that you can't use you can sell on the AH or you can take it to the Obliterum Forge and turn it into Obliterium Ash. You can combine 100 obliterum ash into Obliterum and use that at the forge to upgrade your gear by 5 item levels to a maximum of 850. The upgrade process also requires 2 BoS. So it will take a fair amount of BoS to craft one useable piece of level 850 gear. Note that only soulbound gear can be upgraded although the Obliterum is BoE.

As for dual-gatherers, about all that they can do with the BoS is use them to power the Obliterum Forge. That is, you could send useless gear to your dual-gatherer (or buy it at the AH), turn the gear into obliterum ash, turn the ash into Obliterum and use the forge to increase the item level of the dual-gatherer's soulbound gear. They also said that they may consider adding a vendor for the BoS but it's not clear what you could buy with them.

As to why BoS can't be BoA: "We don't want you to have to have a full stable of alts in order to play the game as effectively as possible." and "Blood of Sargeras is meant to be a measure of how much you are playing the game."

See also Wowhead's guide for Professions.

Here's a good guide (see 2nd comment) for the quest chain to learn the use of the Obliterum Forge. Note that, for the Trial By Fel Fire quest, you will need four level 815 bracers (cloth, leather, mail and plate).