it all depends on the main boss

I'm thinking something that really doesn't have a will of its own. Maybe something created by the new boss? Kind of like Cerberus is to Hades.

yes, it does depend on the end boss. but i think inferno scorpion would make a great boss/subboss. he could really show the n00by new MK characters how its done. as long as the sub boss is better than Noob-Smoke it'll be fine

Assuming the final boss is something along the lines of a reinvigorated Shinnok, or some other type of heavy duty sorceror, I could see the sub-boss being a powerful specimen of the orochi hellbeasts seen in the MK: Shaolin Monks game.

Or if you want to get really insane, how about a cyber-shokan?
You know, with machine guns built into his lower arms and retractable swords in his upper arms!

I know, I know...Lay off the pipe and get some sleep. Hear it all the time.