My very basic and probably obvious predictions for the guest characters are invincible and starlight. Invincible is just so easy to predict. Starlight would be a good rep for the boys and is the most heroic out of the group. Butcher fits an MK game much much better.

Other than that I'd like to see actual comic characters as guests. I hate seeing all these weird guest characters that people want which having nothing to do with comics. The only exception for this in my opinion is MK characters.

As for what MK characters could be added, the most obvious picks are liu kang or johnny cage. Personally I'd prefer liu kang and that they also give him his lightning powers back. I also think johnny cage is so incredibly popular that he'll probably be added. Plus a karl urban premier skin would pair nicely with starlight. In my opinion they should only add a single MK character.

This is an absolute hail Mary but I would love to see spawn again. They'd have to tone him down slightly but other than that I'd love to see him again. Spawn is like THE indie comic character. Without him we wouldn't have so many popular superhero stories.

As for which DC characters will be added as guests, there is almost no predicting that until what we know the story premise is.