Personally, I think we should get one more game out of Onaga. I think it'd be a waste after all the build-up he got. I'd rather they don't make him another Shinnok.

I think it's too soon for Raiden or the One Being to be the boss. Raiden isn't even really "evil" yet and I think he needs another game or two before he can become the boss. Raiden going dark is a good idea, but they can ruin it if they rush it.
And we don't know enough about the One Being yet for he/she/it to be the next boss already.

And I'd rather Shao Kahn not be the boss again for a while. He's had his time and I think others need a chance. And I like how he is now, short on power with no army and trying to regain his throne. I'd like that to continue for a bit. See how far he goes and what he does to regain his strength.
They should at least have him playable again for the people who don't have Gamecube.

Well i don't believe they should use Onaga as the boss, becuase he was already the boss in MK: Deception and is killed in it.

So was Shao Kahn in MK2.

Well, lets say Onaga is the boss, his difficulty level would be greater in the newer game. Plus that would be nice if you could unlock him in the new game.

Younger Subby as a Mid Boss would be cool

People have been talking about raiden going to the dark side, but i don't think he'd be a boss in the next game, maybe a few games later. I would like to a new boss character, like what they did with MK:D, but lets hope the boss character would be unlockable.