Here's how it works, during matches, a random character who's not being used by any of the players appears in the background and he or she will have a task for you to do, and he or she will be an Ice Climber style ally with whoever completes the task first. The character will also use speech bubbles to say what they want you to do, unless the character is known for being silent, if that kind of character shows up, there will instead be a note on the top screen that tells you what the character wants you to do. The tasks the characters will want you to do would be a variety of different things, such as:
Cause 15% damage to each opponent before 60 seconds is up.
Bring the character a Fairy Bottle.
Use you up tilt 5 times in a row without taking damage.
Go 30 seconds without dodging or air dodging.
KO an opponent before 60 seconds is up.
Coins will rain from above. Be the first to collect 30.
Use your neutral special, side special, side special, neutral special and then side special in that order.
Make an opponent dizzy somehow before 60 seconds is up.
Bury an opponent somehow before 60 seconds is up.
Characters who use speech bubbles could also have special interactions with the character who completes the task first depending on who it is. For instance, if you play as Mario and Bowser is the character giving the task and you complete it, Bowser could say "Looks like you and me are gonna work together Mario! Don't say I never do anything for you!"
Assist trophies with extra steps
No joke at all. It is just the equivalent of the FGC' assists.
Way too complicated, interferes massively with match pacing. Also, as others said, assist trophies do this but better
This is different. Your character and the NPC character will become an Ice Climber style duo if you complete the task he or she gives you.
Challenges and dialogue mid-match? This is like the WarioWare stage if it wasn't incredibly unoptimal to not do it and unecessarily complicated.
The WarioWare challenges were at least quick and something any character could do, most of these require items or specific characters. These challenges are way too specific, take way too long, and have too strong of a reward (ignoring the fact you can't make everyone an Ice Climbers style duo without potentially severely messing up balancing and the fact characters have different framedata).
Jamboree Buddies is Smash would be a fun idea, but Smash unfortunately isn't the type of game where something like this isn't really possible without severely messing up balancing and pacing