Also, which one is the classic MIB "straight man"?
Having both Hemsworth and Thompson play for laughs is ... weird. MIB worked because you have the straight-laced veteran and the comedic rookie. It was a simple, effective formula that allowed the two characters to bounce off each other.
IF this movie is to work, it'll have to be because they give us two new leading characters who we find likeable, and whose relationship is interesting. They don't have to repeat the rookie/veteran dynamic of the original films, in fact, they'd be well advised to try something else because they couldn't possibly do that better than the first film!
I just hope they don't try the old dislike-turning-into-love cliché, which is always a temptation when you've got a man and woman working together.