I have no idea. I encountered both Shang Tsung and Quan Chi in the Netherealm when I was on a mission to get the Kamidogu to that realm. Shang Tsung was one of the hardest fights I came across in the Netherealm. I still have not managed to beat him yet.
I remember meeting Quan Chi later on in the Netherealm but our meeting was put to a halt when Scorpion came and chased Quan Chi. I don't remember fighting him though.

Yeah quan chi comes back later after the scorpion incedent. shang was pretty hard. and im still not 100% sure there not playable. mabey there some type of MKDA mode yet to be found or something. there nothing new just carryovers from MKDA.

theres other MKDA who appear in konquest as well. However, the announcer never says their name, they don't have special moves, they don't have a weapon, and lack finishers. It's easy to put them in.... just use their MKDA spirit and their style(s). They're only playable thru cheat devices.. thus far. I doubt they're actually playable.