The big story hype is that we're gonna be seeing characters from alternate timelines and dimensions, so here are my ideas:

Theory #1
Could it be possible that Raiden and the survivors discover a portal back in time to the ORIGINAL timeline somewhere between MK3 and MK4, where he recruits the Earthrealm heroes of this timeline and brings them back to the altered timeline to help defeat Shinnok?

OR!!!

Theory #2
Could Raiden be recruiting heroes from different timelines e.g. the ORIGINAL Kung Lao from 500 years ago, pre-Mythologies Sub-Zero etc.

OR!!!

Theory #3 (my favorite and most likely)
The constant unbalance and "messing with the Furies" due to foreign invasions causes the Earthrealm to become so unstable that it tears holes in time and space all over the world, sucking in characters from all different timelines of the MK series.