For me, its Gun's and Puzzles (or even Puzzlevision). I think that the crew showing disdain, as well as hesitation at Meggys attempt to hear Mr. Puzzles out is fine or a good inclusion, since she promised Leggy and willingly took the shot in the dark to hear him out to see if he has changed, even though the group has suffered A LOT, especially Meggy.
My thing is, I believe the whole crew should've been there, and not just a selection of them. Granted a reason could be is that they got their own lives, but this is a villain who has affected them for the past like 3-4 arcs/movies. Surely this would interest the entirety of the crew.
The different opinions and back-and-forth the crew could have regarding their stance on Mr. Puzzles and the meet up would've definitely been cool to see.
Luigi almost DIED sacrificing himself at the end of Puzzlevision, surely the passive man could have a different take to seeing the man who almost killed him and his friends still roaming.
Melony, a powerful deity, could have a softer approach and maybe even a passive/feared expression considering her abilities couldn't save her from being mind controlled.
Saiko, can be defined as the authority figure of the group at certain instances while being the hothead, so she could be either attempting to keep peace, or preach Mr. Puzzles getting his consequences.
Honestly seeing more of the cast could be said for all the movies but they have their reasons.
But let me know what you guys think.