The more that I think about it, the more that I think that Liu Kang and Kung Lao are more likely as Injustice guests over other MK characters.
The top 5 most popular MK characters seem to be Scorpion, Sub-Zero, Raiden, Liu Kang and Kung Lao... and I'm going not only by the amount of in-game appearances but also other non-fighting MK games and merchandising stuff.
Sub-Zero had his own Mythologies game.
Scorpion has his own solo movie.
Liu Kang and Kung Lao had their own action-adventure game.
Scorpion and Sub-Zero are unlock-able playing characters in Shaolin Monks after beating the game once each with Liu Kang and Kung Lao.
The only 4 klassic characters to have their own klassic Pop figures are Sub-Zero, Scorpion, Raiden and Liu Kang.
In one of MK11's ending Liu Kang chooses Kung Lao to be part of the Elder god council.
In Liu's Aftermath ending the Great Kung Lao is chosen.
Dunno, maybe I'm over-thinking it, but I think Liu Kang and Kung Lao have the best chances as DLC for the next Injustice.