Honestly, I just want Jade to return.
However, I'd be super happy if Reiko, Nitara and Rain returned as well.
Reiko is most likely in the Netherrealm... maybe he has somehow kept some of the blood magic he obtained in the MKX comics and are now aiding the new king and queen of the Netherrealm as an advisor or general.
I can see Nitara helping out fighting Kotal Kahn, considering he is all about keeping the realms merged with Outworld while Nitara is all about her realm being freed from Outworld. Maybe she will join up with the Edenian Resistance or the Earthrealmers.
I loved Rain in MK9 and MKX, but was sad he wasn't playable in the latter. Not sure what he will do this time around though, now that Mileena is no more... and something tells me that Tanya won't return, since she was only DLC and not on the main roster in MKX.