Since the Forsaken can not reproduce and Sylvanas's is running low on valkyries and her plan in Stormheim being thwarted, therace she leads might be facing extinction...
Now there a few different ways of how it might go from there like people saying she is gonna become a new Lich Queen since the Lich King himself can raise dead. But the other which I think would be interesting is that she might be rewarded for something she is about to do in the future or attain the power herself to give her people flesh making then literally humans again... This wouldn't change anything in Forsaken being part of Alliance, they will just be humans again solving the issue of Forsaken's extinction... They might add a new racial as a flavor similar to that of a worgen to be able to transform from human back to undead and vice versa (even though lore-wise this wouldn't have been possible since they would be given a flesh, I'm suggesting this just for the gameplay reasons since Undead would be a playable race for like forever by then). As for two human races being in a different factions I don't think would be much of the problem since we see that happening even right now with blood elves and high elves which are as well the same race...