MK3 introduced Animalities, which were rumors. UMK3 introduced Ermac, who was a rumor. MK:A introduced Blaze, who was also a rumor. MK9 introduced two:the one where you could feed your opponent to the trees in the Living Forest as a Stage Fatality, and Skarlet as DLC. Now, Ed Boon could create a premise based off of a rumor from the very first MK in MK10.
I shall summarize it in just two words:Nimbus Terrafaux.
If Netherrealm can't do that, what will be the point of buying MK10??
Other rumor characters that could be introduced:
1.Aqua, the black and slightly yellow palette swap of Scorpion who was rumored in MKT.
2.Hornbuckle, the green palette swap of Liu Kang seen with Blaze in MKII in the Pit II arena.
Any other rumors you guys want, say it in the comments!