Take or make a generic dungeon. Reuse one you’ve already done and reskin it.
add 4 other adventuring parties exploring it.
The 1 in a million chance. Not only is then another party there’s 4!
roll for their dispositions. Hostile to friendly. Maybe one attacks, one talks, one suggests teaming up!? etc
Have they cleared a room or two? Roll to see if they already got the treasure of some of it?
If you want to add complexity roll to see how they’re exploring the dungeon as the players do.
They could meet up after the initial meet and one or some of them were killed.
They could interact with each other. The players meet both groups and later in the delve find that one group killed the other! Your players come across a room with their bodies stabbed and looted!
Have a big enough dungeon for this but it can have more than normal empty rooms and increase the chance of meeting the other parties.
You can have fun and plan things out. Give the different parties themes, flavor. etc
but this can also be used with minimum prep of rolling to generate them and see what happens!