Let's assume that Sylvanas and Nathanos, whatever happens to them, won't be returning to Azeroth or the Forsaken (and thus making their abandonment of the Horde impactful and last for longer than two years).
What's going to happen with the Forsaken?
Calia's involvement hasn't been shown to extend to the Forsaken as of 8.3: while she absolutely could lead them moving forward (something something Lordaeron) she is only seen taking in the Undead Night Elves, and it goes against Ion's one comment on her that she isn't Horde. Voss is the ambassador of the Forsaken on the Horde council but other than her there are only a few notable Forsaken left that are friendly, and even less that are actually the "Undead" race (what is it, Voss, the Apothecary and Belmont? we don't even know who is sticking with the Horde besides Voss)
Maldraxxus seems to be a hint on where they could be going (Scourge aesthetics but friendly and apparently noble) but they could always use Calia (and Derek) to give them a new makeover. Will they keep the purple Tim Burton aesthetics and Sylvanas-themed flag? Will they still have blight everywhere? Or will they lean onto the Maldraxxus aesthetics or the Lordaeron aesthetics?
Alternatively they could be basically ignored forever here on out but from Ion's comments that doesn't seem to be the case. Interested in all your thoughts and replies that are akin to things like "the Forsaken should all be put into a rocket and shot into space".