The Forsaken could act as a mediator between the San’layn and the Blood Elves in order to get them to learn the true meaning of Winter Veil and accept them.
Right now, popular opinion of the San’layn by the Blood Elves is being very dubious torwards them. Moreso with the recent Blood Prince that caused problems in the Blood Elf heritage questline that we had to beat up and blow up. So they would be generally wary of accepting them again. Of course, this is a bit hypocritical of them since they allow Darkfallen and Death Knight Blood Elves just fine and even allow Human zombies walk around their city without much of a thought. Makes me wonder what their thoughts were when some San’layn tried to join the Horde in BfA.
I don’t see why the Forsaken wouldn’t be sympathetic to the plight of the San’layn. Especially when they have Darkfallen within their ranks now. It wasn’t really their fault they became San’layn and they’re obviously not in the best situation at present. Besides, if the San’layn can take control of the vampyr curse, much like how the Worgen took some control over their curse, it would mean that they wouldn’t continue to grow in numbers and be a potential threat in the future. Plus they would make for some really awesome allies.
The San’layn that are able to gain some control over themselves again have an uphill battle in proving themselves as they fight off the vampyr curse they have. And frankly, that part of the curse should be weakening enough (Lich King’s Death, Crown of Wills, etc) for them to become more like themselves, which would play into their storyline quite well.
Finding themselves undead suddenly with a curse and possibly addiction they’re trying to fend off isn’t exactly the ideal fun vacation time. There is also the matter of several of them being followers of Kael’thas at one point, which of course they became San’layn because of him and Kael’thas would’ve likely betrayed them as he did later on anyway. Both of which can work well in their storyline.
We don’t know what is going to happen in Midnight, but there has been hints of void stuff and it has been brought up that San’layn could have some power over the void to help give an advantage. So in terms of “uniting the Elf Tribes,” Horde characters could work on getting the San’layn in control of their vampyr curse while helping the Forsaken mediate between them and Silvermoon, eventually letting those San’layn accepted back into Silvermoon and becoming full members of the Horde.
And of course Ghostlands is getting something of a revamp, at least to allow flying, so you may as well design Deatholme to be the racial hub for Horde San’layn. But as I said many times before, I don’t want to count my chickens before they’ve hatched seeing as missed opportunities tends to be a trend sadly.
Still, it’s worth bringing these ideas up in my opinion. 
–Still dreaming of playable Horde San’layn as a full race! 
