Just thinking a little bit here, what if Desmond becomes the new MIB. Of all the people in the alternate universe, he seems to be the one who has lost the most. He no longer has Penny or his son. What if all that he's doing in the alternate universe is so that he can get his other life back? And what if he's punished for this misdeed by being doomed to be the MIB? That would be a good twist on his supposedly good intentions...
I don't think that's what's happening. He seemed pretty happy in the Alt before he got his new understanding. And Penny ain't dead. All he has to do is ask her out, if he wants to start something in the Alt. In this show, anything's possible, even the ridiculous, but I don't think that's where we're going (and I don't mean to say your idea is ridiculous)
We are all waiting for Desmond to be the savior of BOTH universes, when in fact he could be MiB himself which somehow managed to cross over from the original universe to the sideways via Desmond and the Light...
And at the end, he kills them all in both universes!
Priceless...
Hey, Evil has to win some times in movies and tv shows!
If there is a new MIB, then my vote would be Ben. Just the right attitude. I think Altverse Desmond knows IslandDesmond and that is why he acts the way he does.