When you consider that all the other humans were either killed (Sub-Zero, Jax, Kabal, Shang Tsung, etc) or converted into cyborgs (Cyrax and Sektor) while residents of other realms have longer lifespans, there's a way that they can reappear 25 years later without aging much if at all, there are still four fighters from MK9 who may be way, way too old during the second half of MKX.

Granted, I can still see Kenshi as some sort of old blind master in the future but as for the other three, I'm not sure if they'll work as 50 and 60 somethings and Casie Cage has a Johnny fighting style and a Sonya one so she can easily carry the torch. The one fighter I'm scared about losing permanently is Kano. Kano has one of the most unique fighting styles since he's a hybrid zoner/grappler which is next to unheard of but was bottom tier in MK9.

Keep in mind that all of the deceased fighters from MK9 can be resurrected due to Quan Chi's bargain with Shao Kahn so nearly all of them can return and most of the Earthrealm characters from the 3D era were never all that great. I just dislike the potential loss of anyone with Kano's moves after MKX ends.

As far as characters who will probably never appear in the new timeline, I don't expect Kobra, Jarek, Hsu Hao, Shujinko, Frost, Kai or Mavado to appear ever again. Keep in mind that I like the latter three but Mavado would never work in a 2D game, three fighting styles for each fighter make Frost vestigial and Kai's overhead fireball was given to Ashrah which is all that Kai had to make him stand out anyway.

Keep in mind, that I fully expect most of the cast to be new and the dead fighters from MK9 to reappear in MKXI.