I'm not sure if this fits into the category, but I'd like to see a common plot line.
I love the multiple plot lines in MitM. But I'd like to see one ep in which they all (not just the boys) work together. Sort of like Family Reunion and Krelboyne Picnic, only to a greater extent. It'd be interesting to see (or, at this point I suppose confirm would be better) exactly how everyone fits into the family.
And since those 2 eps were in earlier seasons, it would also tell us more about how the characters have changed.