On elevators to the same next level, "open to all" / "unlocked" parties with less than 4 members should be checked, if they are merge-able.
Examples:
2 parties of 2 should join into 1 party of 4, randomizing the party leader.
1 party of 3 and 1 party of 1 should join into 1 party of 4.
3 parties of 1 should join into 1 party of 3.
...
Maybe an option during party creation should control this feature.
"Go solo" would probably be no problem, because it's unlikely for split parties stepping on the elevator at the very same time.
Merge should take place during heat animations, "up"/"down" button timeout and elevator moving down animation.
What does this solve other than getting certain other random players joining instead of ones from the Arcade?