Yeah, your probably right about Goro. Actually, I like the idea of Goro being a playable non-boss character. More balanced. I hate when playable characters are big bosses that are just cheaper, OP versions. Bosses should be designed AS BOSSES.
I'd have to agree about Motaro being a sub-boss. I doubt that Moloch is coming back considering he was lame as shit, and Quan Chi kills him at the start of every fight haha.
I wonder about the other more obscure characters like Belokk, or some of the enemies from MKM:SZ. A sub-boss inspired by some of the mecha enemies or the dudes who ride on dinosaurs would be awesome. Not just one of them, but more like a Brotherhood of Shadows general. A more intelligent oni that has a velociraptor mount. I guess at that point you might as well program Motaro. Way off topic now. So, yeah... how bout that Onaga?