People need to get it out of their heads about unplayable bosses. I believe they only did that in MK9 for nostalgia reasons because that's the way it was back then. But look here: playable Goro. Injustice's boss was playable.
And unplayable bosses have a tendency of being unplayable when they are big brutes (Goro, Shao Kahn, Motaro, Moloch), while normal-sized bosses have been playabke (Shinnok, Shang Tsung and Quan Chi). Last I checked, Liu falls in the latter camp.
And it would be a huge dick punch to have one of the most popular and iconic characters in the game, but unplayable. There would be a lot of backlash over it.
Bottom line: If Liu Kang is a boss, he'll be playable.