I really doubt Nomura would use a series like The Simpsons or Family Guy lmao. I think that's a level of ridiculousness I don't think even he would want (nor would any of us, unironically).
Avatar would be a cool fit, but other than that... Nah, I don't think it'd ever happen. You're more likely to see the Avengers in a KH game, and even that might take a lot of work to implement. Which is funny, because Sora's abilities fit in pretty normally with Marvel's whole universe. People think it'd be weird, but it actually works better than you'd think. If we're talking post-KH3 Sora, he'd probably be able to solo the whole Avengers team himself. BUT, that's another topic entirely lol.
Star Wars is also one that I feel like it would work excellently, and I'm surprised people think that one wouldn't work either lmao. "May your heart be your guiding key" is quite literally the KH-version of "May the force be with you". Keyblades and Lightsabers are both mystical weapons that not just anyone can wield (theoretically though, anyone can use a Lightsaber, but you'd have to be Force-sensitive to use properly... I think that's how it goes?). The Jedi and Sith are similar to the Guardians of Light and Seekers of Darkness. There's so many reasons as to why this would work, and I actually would love it if Nomura ever decides to do it.
In the end though... If you asked people in 1999 if Final Fantasy and Disney would ever mix, 99 out of 100 people would've told you no, and yet, here we are in 2020. There's obvious limits of course, but even then, it all depends on the execution. The entire Kingdom Hearts concept could've been a huge misfire had it been done by anyone else, so KH1 really had to nail it for the series to become what it is today. And thankfully, it did. So I mean, you never know, it depends on Nomura's creativity, vision for the series, and whether he actually wants to do it or even likes those properties that people suggest.