Unfortunately it is anyone’s guess as to who these new characters could be. However it has been speculated that the new characters could be part of the Kombat Pack 2, which is basically a DLC. However given that neither Boon nor the studio have actually confirmed anything yet, safe to say it could be anything.
However Boon did tease the announcement would be coming soon so maybe we will learn more about it later today. That being said, some fans have speculated that at least one of the new characters could be The Pit from Mortal Kombat 9, and that another could possibly be Nightwolf, a fan-favorite from some of the older Mortal Kombat titles.
In fact Boon’s last tweet was that of a moon, and given Nightwolf’s ability to shapeshift into a moon, like a werewolf in full moon, that could be it. It does feel like we’re reaching here so take it with a grain of salt, but hopefully all will be revealed later today.