If Gumball or Regular Show appeared in Team Teen

Well Gumball would have some lamp-shading humor. He could remind some of the characters of the fusion fall game. That would be an awkward moment for Steven. You could use The Void to show retcons, long lost characters and other stuff that was erased from other shows.

Considering both those shows are full to the brim of weird events, Team Teen could go there to investigate. 

You should add Rick and Morty.

The only way I can see 'Regular Show' being involved was if a villain in disguise was part of an influx of new workers at the park, and the team came to find this villain, only to be aided by the shenanigans of Mordecai and Rigby.

For Gumball, I could see a situation where Kenneth the gross jar monster got out and is wreaking havoc on the town of Elmore.  Gumball and Darwin are panicking and handling the situation only as a Watterson can until Team Teen shows up to help them.  An alternate idea could involve Dr. Wrecker, but that would be more a comedic story.

I don't actually consider any of these shows as suitable for Teen Team, but if there's any posibility to add some characters, I'd prefer Gumball and/or Mordecai as the "comic relief" of the group.

While Gumball & the Regular Show cast have saved the world multiple times, it's usually because they caused it. Plus, they don't really see themselves as heroes or noble helpers and are usually dragged into adventures.

I imagine that the Team is going to ask help from Skips or Anais and Gumball/M&R interrupt and somehow end up going with the Team. Once again, they're dragged into some sort of adventure and are pretty much stuck with the team

Gumball could just suddenly show up to them. Lets say they were fighting a villain and, when they beat it and celebrate, Gumball falls down from the sky. Sounds random at first but then he explains (in flashback of course) that an event(s) that happened in Elmore made him end up there. What the event(s) could be is up to you like maybe a portal opened like the void did or a huge explosion happened sending him across the world.

Regular Show would most likely be the team visiting there world instead of them going to the team. They follow a disturbance all the way to The Park where Mordecai and Rigby are working or something along those lines.

That would be cool.

This is a suggestion but, maybe Team Teen needs some sort of help or advice which Skips could help them in some way, then inevitably Mordecai & Rigby butt in because their big superhero fans or something like that, but they both accidentally cause trouble for the team to fix before Benson finds out?