Liu Kang and Raiden have been around since the first Mortal Kombat game, but the ending of MK11: Aftermath shows having both characters in Mortal Kombat 12 would be unnecessary. After defeating Shang Tsung once and for all and remaking Mortal Kombat's timeline Liu Kang has become a god himself, and he is the one who should adopt Raiden's former "Protector of Earthrealm" mantle in future games.
This is, of course, assuming the next Mortal Kombat game follows the canon and story of MK11. While unlikely, it is entirely possible NetherRealm Studios could take the Mortal Kombat franchise in a wild new direction that includes none of the characters, plots, or setups from previous entries. Based on the series' record, however, this probably won't be the case. While convoluted and lengthy, Mortal Kombat's canon has been (mostly) consistent since the early 1990s, and there is no reason to think something as simple as completely rewriting the MK timeline will do much to get rid of certain series staples.
If Mortal Kombat 12 does indeed follow from the "good" ending of MK11: Aftermath, then players will likely get to see at least Liu Kang, The Great Kung Lao, and Goro as returning characters, and while Raiden could theoretically appear in this period, it would be better if he didn't. The past three games in the MK franchise have been heavily focused on conflict between Liu Kang and Raiden, and seeing the same arguments play out once again (although this time between two gods, not just one god and one mortal) would become tiresome fast. It would be much more satisfying if NetherRealm retired the Thunder God from Mortal Kombat and Liu Kang replaced him as the overseer of Earth's warriors when MK12 releases.
Mortal Kombat 12 Fighters Need A Leader
While the full roster of Mortal Kombat 12 characters likely won't be known for some time, ending MK11: Aftermath with Fire and Thunder God Liu Kang meeting The Great Kung Lao offers a lot of interesting possibilities for potential warriors. No matter who is included in MK12's final roster, Earthrealm's heroes will need a mentor figure to rally behind. In the original Mortal Kombat games this role fell to Raiden, and in Mortal Kombat 12 it should be Liu Kang.
This shift in roles is already hinted at by the end of MK:11 Aftermath, as Liu Kang tells his ancestor "I have chosen you as my champion," and insists he must be trained. While the game doesn't offer a concrete answer on what exactly Liu Kang wants Kung Lao to train for, it can be assumed that it is for the next Mortal Kombat tournament. Having Liu Kang refer to The Great Kung Lao as his "champion" is exactly the sort of language Raiden would have used, and insinuates Kung Lao will be the new Liu Kang-esque character in the MK series moving forward.
Raiden was an important part of Mortal Kombat's history, but also a destructive one. Although he began as a good-hearted deity who only wanted to protect Earthrealm, time and again he fell victim to both Shang Tsung's schemes and his own evil urges. Players have seen all sides of Raiden's personality, and now it is time to move on (or, as MK11's ending suggests, backwards) to a time without Mortal Kombat's Thunder God. Including Raiden in Mortal Kombat 12 would likely please a lot of people who view him as their favorite character, but for the sake of Mortal Kombat's story it would be better if he stayed out of the fight.