Deathstroke and Black Adam for sure.
Tryborg might take inspiration from Brainiac and Ultron, if he'll ever return.
For the rest...naaahhhhhh.

Despite DC and Marvel popularity and their (minor) role in creating what Mortal Kombat is today, MK should focus more on the kung fu flicks, the breeding of eastern and western culture and tropes, with minor horror and fantasy elements.

MK is a beast of his own, there's no need to take characters from other franchises. That's why, despite their good sales, guests don't work in MK, but, ironically, could work in Injustice.

If there's a game that needed 4 guest characters, that's Injustice, not MK X.

• Black Adam - great choice! The parallels here are obvious. Joker even looks at Black Adam and asks if he's had a chance to "meet that other lightning guy" - a clear allusion to Raiden!

• Deathstroke - this guy was in MK vs. DC already. I was considering DC characters who haven't really been a part of the MK universe.