"I might just!" leaves out some verb. You can tell from context. Indeed, to say this statement, there must be a context. Someone else must have said something and you are replying. A: If you are going to be mean, you might as well leave! B: I might just! (I might just leave!) A: If you don't want it anymore, go ahead and throw it out! B: I might just! (I must just throw it out!)