Xal'atath, the final boss of BFA.

[Jastall]

Old Gods are almost the size on continents, Yogg is said to be almost the size of Northrend. What we fought were mere physical manifestations of them, nothing more.

Their total size maybe. We still kill them by whacking a strategic spot with swords and fireballs well enough, and the most powerful of them all was as nothing compared to a Titan, and the assembled Titans got rekt by Sargeras. The Old Gods are not even on the same power scale as old Sargy, whereas we at worst need a bit of NPC help and we can kill the likes of N'zoth without much incident.

[Khaza-R]

Doubt it.

Xal'atath will probably be an ally in a Void/Light expansion

[Powerogue]

Eh she comes off more as the manipulative schemer type. If I had to guess it'd be more like Locus Walker being a semi-evil mentor figure for Alleria.

And hey, since her corporeal form is a void elf, it'd sure start some nice drama if the alliance's void elves let her join their club. You Hordies are hogging too much of the evil, having an actual old god on our team should even those scales.

[Nirn]

Yeah... at some point I kinda just defend my characters actions as I've killed 1000's, if not more and my morality no longer exists. Get quest, I no longer read description, kill whatever is red even if it's cubs or children, collect reward. I'd imagine some of the adventurers don't care anymore about good or bad, light or void.

Pretty much.

At this point, if you look at the sheer number of species that we have been exterminating for some shiny loot, one has to wonder if the adventurers are not complete and utter genocidal maniacs, doing it purely out of sadistic joy. Hell, even Xal'atath says this to you at one point

You were a good choice. No other wielder has murdered their way across a continent like you have. I admire your endurance.

[Foolicious]

With the recent way blizzards tells their stories i doubt we see her again in bfa. Remember the last time we get to know about an unknown deal between characters?
There are so many storylines that get started and finished in another expansion, or sometimes never. We waited nearly three years to see yseras death being briefly mentioned. Locus walker still has some unfinished business and a lot of us thought he was a major story character. And the list goes on.

Also it was mentioned in an interview with taliesin and evitel that the final boss will be hinted in the dungeon journal of azshara and will be revealed during the fight itself.
So its most likely n'zoth, magni or azeroth herself depending on how much you want to read into voicelines.

[KainneAbsolute]

I think Xal took a portal to a galaxy far away...

[Amaterasu65]

I think she's gonna find ways to empower herself so she will never be bound or enslaved again. After this happens she will be put to the sword in a void themed expansion/raid.