At this point, given MKX's story and who we got as KP characters and what they dealt with in the story in MK11 I think it's silly to expect and request for the likes of Onaga, Nitara and even my own favourite Havik. I rather they give them story relevance, but since it could be the last MK game I do understand. I think a reasonable list from my side of who I think would fit and could actually make it - Ermac, Reptile & Smoke 100% won't be back - I'd like:
Kai - was considered for MKX, could be given a surviving the Geras attack on Wu Shi Temple or upon hearing it return to fight and would have a very unique skill set
Sareena - was in MKX, a demon who worked for Quan Chi could be given some of his moves and some unique devious qualities
Kung Jin - was in MKX, a guy who seeks redemption for his past mistakes and brought in upon Kotal's request as he remembers the humiliation. Could make some nice back and forth between him and Kung Lao as well as Raiden.
Havik - was in MKX comics, easily the most creative guy you could invent moves for and is mentioned quite a few times and with Joker being in, it has opened up the door for his return. I'd rather he'd be given story relevance, but I don't see him returning any other way.