it would be great to have Michael included off camera. Have something in the vows be borderline and then you hear a shout out of "that's what she said." But, they don't have to show him. Just know he was there and still the same would be enough.
That would be funny, but I think TWSS was put to rest perfectly in the Goodbye, Michael episode. When MS is at the airport and remember to take his microphone off and he's like "It's gonna feel so good to get thos off my chest.." and then unmic'd he says TWSS and walks off. Classic Michael.
I think he'll have a small part in the finale, but the show has moved on now. So he shouldn't be anything major as far as plot. Also, I assume that since they'll be showing "6 months later" versions of everyone, that we could potentially see Michael and Holly with-child.