The first thing that came to mind... which honestly was nothing like Kingdom Hearts, is
Captain N: The Games Master. A cartoon about an '80s youth who goes to Videoland where he visits other worlds based on NES games. Though keep in mind given video games lacked personality back then most of the characters and worlds they belong to were mostly parody versions of themselves, such as Simon belmont being narcissistic and a bit cowardly.
Serga Hard Girls is perhaps the closest equivilent to a Sega version of Captain N even though given it was animated in MMD means it's on a more limited budget and each episode 11:30 long. It follows the same formula of the three girls arriving at a school which consist of a lobby and a class room and they go on an adventure inside various Sega games.
As for Disney related crossovers... eeeh
House of Mouse comes to mind. Unlike Kingdom Hearts where Sora goes to worlds based on Disney movies and have fantastic adventures, here the characters from other Disney movies go to this nightclub owned by Mickey Mouse for entertainment and cartoons. Each episode consist of a story of Mickey and his friends managing the nightclub while usually some problem arrise such as a problem with maintenance, Pete the landlord wanting the club shut down or other problems, all the while Mickey has to keep his guest entertained and happy with various cartoons. Sometimes guests get involved with the episode but House of Mouse isn't as grand of a crossover adventure as Kingdom Hearts.
As for
Descendants and
Once Upon a Time I never watched those Disney crossover shows so I wouldn't know. But given Disney's been around for 98 years I wouldn't be surprised if I miss anything.
These examples aren't as grand crossovers as Kingdom Hearts but they're all I can think off from the top of my head. That and
Disney Infinity which was a crossover Disney game that used figurines that came to life like what Skylanders did. However
3.0 was the last edition back in 2016 so it's an old game. A missed oppotunity to have Sora in Infinity.
