Multiversus' director has said that he's going to "try really hard" to include Mordecai and Rigby in the game's roster.
Multiversus is absolutely stuffed with characters from across Warner Bros. different properties, including DC, Looney Tunes, Hannah Barbera, and Cartoon Network. Although a good deal of its most popular characters are being represented in the game, there are still tons to be added and plenty of fan favourites that are constantly being requested. Out of all of those possible characters, Regular Show's Mordecai and Rigby are by far one of the most requested, something that the team is clearly aware of.
Tony Huynh, the director of MultiVersus, regularly replies to fans requests and questions on Twitter, usually about which characters can and can't be in MultiVersus. Yesterday, Huynh was asked by a fan about including Mordecai and Rigby in the game, to which Huynh replied with, "I'm going to try really hard".
Considering Huynh replies to a lot of these requests on Twitter, and sometimes pretends he doesn't know whether a character is in the game like with the Iron Giant, his response here is different from usual. Huynh usually takes these requests and says things like, "that'd be cool", or "if the licence holder will let us", usually keeping it purposefully very vague so as to not promise anything.
What makes this response a little more interesting is that Huynh outright says that he's going to be trying to get them into the game, clearly recognising how popular of a pick the duo is. However, it's worth noting that it's pretty clear that Mordecai and Rigby aren't currently planned for the game in any capacity, as Huynh implies that it's something that he's going to work on.
Datamines have also revealed pretty much every character that might make their way into MultiVersus and, sadly, Mordecai and Rigby haven't been hinted at once. Some of the leaked characters like Static Shock and Craig of the Creek only have one or two voice lines suggesting they're going to make it in, but they at least have that one voice line, something that Mordecai and Rigby don't.