I doubt any of the Kameo fighters would be guests from other IPs. They'd want the guests to be fully fledged playable characters. (Unless Guests come with their own Kameo fighters...which could actually be dope. Imagining Homelander being a fully fledged guest character with A-Train, The Deep, or Queen Maeve as Kameos exclusive for him.
But overall, I don't understand the hate for this mechanic.
"B-B-B-But- I'd rather have Meat be a playable character!" Well, the chances of him being in the game were slim to 0. But this would be a way for characters that had 0 chance of being playable, to actually be acknowledged by the developers & maybe even gain some popularity amongst the general audience (increasing their chances of being fleshed out playable characters in future installments.)
I hope the gameplay without Kameos is fun, with adding Kameos making the experience that much more exciting and fresh. I want the Kameos to actually have a presence in the gameplay. Unique grab animations, Kameo brutalities, Kameo fatalities, intro & outro animations featuring the Kameos you choose, etc.
But I also want the Kameos to have WEIGHT to them. The assists in the MK11 towers were okay, but pressing the right thumbstick up and watching Kitana jump onto the stage just to the throw a fan and jump back out felt very...unsatisfying. So I'd hope the changes the Kameos bring hold a higher impact in the overall matches.
But at the end of the day I really just hope the mechanic is fun and well thought out. I hope the playable roster of characters are fully fleshed out with ALL of their moves, and the Kameos should just add a bit of extra flavor to that.