As other people have noted, Blizz has learned to seed races ahead of time. That does not mean that we aren't going to receive two new races next expansion, only that the options presented make it unlikely.
Truth be told, there aren't that many plausible options either. Ogres are the best remaining choice. I could see arguments made for Furbolg. After that, it gets iffy. Naga present several model problems, Blizz has expressed regret for neutral races so the Nightborne are unlikely, high elves are out, arrakoa would be an expansion much too late, etc.
Basically, ogres or furbolg, and neither have quite the draw that previous additions had. I'm banking on another hero class for the next xpac, or an incredible feature to make it worthwhile (our own transportable ships, maybe?).