Just a a little thing for fun. Preferably, they should be from different movies/shorts, but there might be some characters within the same movie that just don’t meet each other.
I’d want to see Helen meet Randall. I’m convinced a fight between them would be awesome.
Carl Fredricksen and Geri–They’re both old men; I can picture them trying to challenge each other in a game of chess! (reference to running gag in fan-fic)
It’d be interesting to see Wally B. cross paths with the ABL charries… I’m guessing he wouldn’t be overtly thrilled with Heimleich’s portrayal of “cute little bumblebees” 
As far as Pixar characters I’d like to see meet, I once thought of EVE meeting Dory, just because they’re both from movies written and directed by Andrew Stanton. Or even Russell meeting Violet, Dash, and even Bonnie!
JustSoWall-eCrazy: I don’t think it would be possible for EVE and Dory to meet… 
Evspixarfan2012 - Meh, if these had to be possible that’d rule out most of them.
I’d also like to see Muntz meet any Monsters, Inc. or Toy Story character. I’d want to see if he’d lose focus on Kevin if a bigger discovery presented itself, if he’d try and go after both, or if he’d just continue with Kevin (it’d probably be the last one).
Everyone and anyone in the Incredibles, but the first on my list is Violet.
I’d like to see Russell meet WALL-E. <3
As long as this is just for fun, and not head cannons. I would be interested for violet and Merida to interact. I would also like to see Helen and Mirage have a real conversation. I know this happened in the comics, but I don’t consider them canon.
I’ve got another one:
What about when Dug meets Archie the Scare Pig? Then they would play together! (Refer to my fan fic series so you’ll know why I picked that.)
^That would be a cuteness overload right there… might need to draw that or something! 
OWP: It would be more of a triple cuteness overload if WALL-E was with them. (Oh, and I do not know why, but lately and for some reason, I’ve been trying to imagine them pulling a sled-wagon. What a weird thought! 
)
^Sounds like a Christmas card design that needs to be made! 
I’ve got another one, also based on a running gag in my fan fic:
What if Mater got scared off by a “ghost light” that’s actually coming from EVE’s scanner?
Anyone else going to respond? 'Cause it’s been like, 2 months already. Oh well, better get this topic bumped.
Anyway, I’ve got another suggestion (or what do you think of my last suggestion?):
Mini Buzz hanging out with Tinny during playtime (based on yet another running gag in fan-fic)
The ant queen and Elinor. Alternatively, Elinor and Django.
I’m not sure how the sizes would work out, but whatever.
Another suggestion based on fan fic: What about if M-O was friends with BURN-E?
(What? They’re both single units which means there’s only one of each for those two…)