I think a lot of that has to do with the future design philosophy when it comes to customization options, and that’s a bit tough to peg down.
Bear with me, brevity is not my strong suit, I’m afraid 
Alright, so let’s entertain the notion that Venthyr are 100% confirmed as a future race. Would they be allied or core? Well, it’s possible that there’s no longer a distinction between the two. If I recall correctly, the images of the new race selection screen on the alpha don’t differentiate between which races are allied and which races are core; they’re all just sort of thrown together on the same screen. If we have to make an arbitrary distinction for our purposes, then I suppose we could say that allied races represent subcultures of core races that require more attention (read; unique racials or class options) than mere customization options can allow for. So a forest troll can be represented with green skin on a jungle troll, but a Zandalari troll needs to be its own race due to its dramatically different posture, class options, druid aesthetics, and customizations. Vulpera, however, are an interesting beast. Gameplay-wise they are the allied race for the goblins. Lore-wise, however, there is nothing linking the two together and, aside from a shared skeleton and similar heights, the two races bear very little resemblance. So, could Venthyr be treated as an allied race for the undead? Could they be treated as a new core race? My answer is… it doesn’t matter. We’ll circle around back to that, but in my opinion the simple fact that they’re being added after BFA likely means they will be subjected to the new design direction that governs core race customization options.
What do I mean by that? Well, let’s look at the evidence. All racial starting zones are optional now, and the new customization options on alpha let you represent the styles of racial subcultures that might have been allied races if we really wanted to stretch the system that far. What this means is that when you choose your race you are no longer strictly locked into playing as a member from a specific location. For instance, we used to differentiate the two kinds of human by calling one “Stormwind human” and another “Kul Tiran human.” That’s no longer valid as the core race human no longer necessarily comes from Stormwind. This is the entire reason for the new “Exile’s Reach” experience.
So let me cut to chase; There exists a possibility that San’layn won’t be added as customization options for blood elves, but rather as customization options for the venthyr.
How do we know this? Well, the more I read about Revendreth’s lore the more interested I become. Firstly, it looks like the venthyr function in a very similar manner to the kyrians in that “redeemed” souls have a choice to literally become venthyr. Kael’thas is a prisoner and isn’t “redeemed” when we find him, which is why he’s still a blood elf. Uther, on the other hand, is a full-blown kyrian in the datamining so far. As far as we know the other two covenants don’t operate this way; Ardenweald is about rebirth, so it more or less regurgitates the souls that come its way back into the world as opposed to keeping them in the realm. Maldraxxus seems to value individuality greatly, as Draka, Kel’thuzad, and Lady Vashj are all denizens there but none of them have been fully transformed into necrolords and more or less resemble the forms they had in life.
Given this, we can draw a pretty important conclusion; It is entirely plausible for venthyr to have once been san’layn before arriving in Revendreth after their death. In fact, it’s probably highly likely that most San’layn found their way to Revendreth considering the fact that being undead usually means your soul is “damned.” And if we consider damnation in WoW lore to be analogous to “sin,” that would definitely fall under the territory of the venthyr covenant.
So I can actually imagine a scenario where venthyr become a new playable race, but thanks to the new customization option overhauls they can choose to better reflect various races that became venthyr. Maybe, once venthyr start leaving Revendreth after the massive revolution against Sire Denathrius that we take part in, those subdivisions start to become more prominent. So maybe, by the end of it, Prince Renathal represents the venthyr’s overall racial leader, but Kael’thas would be to the venthyr what Falstad is to the dwarves; technically also part of the same race, but using noticeably different and distinct customization options.
So here’s my personal ideal scenario, and this will basically function as the TL;DR of my entire essay; Venthyr become a new playable race. You get vampiric racials and a fitting voiceset to better represent the fantasy. You then get the option, through customization options, to alter the appearance of your venthyr so that they resemble the san’layn, and canonically they would be san’layn. Kael’thas is present as a major lore figure to better bridge the lore between the san’layn and the venthyr, effectively serving as the Horde’s version of Umbric (although hopefully he isn’t ignored as much as Umbric was).
