In my opinion most of the characters that came after MK3 failed big time. A lot of them where not original enough, especially characters who stole most moves from classic characters.
If they release a couple of those post-MK3 characters by DLC my choice goes to:
Considering there is a DLC slot, maybe some possible DLC characters would be either sub-bosses (Goro, Kintaro, Motaro) and Shao Kahn assuming they're not playable from the start.
And then from there, maybe we can see some relevent post MK3 characters.
1. Quan Chi - well he has existed prior to the first 3 MK games so if we have him as DLC, his story could tie in to the story of this game.
2. Shinnok - same as Quan Chi.
3. Fujin - same as above.
4. Kenshi - he's probably one of the only new characters from MKDA and on that has been pretty important. He did exist before MK1, maybe with the altered storyline, Raiden would be able to make Kenshi one of his chosen warriors? (assuming these DLC characters have some story stuff tied with them)
5. Tanya - She seemed pretty significant during the post MK3 storylines and she could tie into this altered MK1-MK3 storyline. I think she has the potential to be a DLC character too.
6. Bo Rai Cho - From what I can remember, he helped train Liu Kang for the the tournament right? I wasn't too fond of him as a playable character but story wise, it actually makes sense to see him.
