You did this to me on purpose, didn't you. I can tell.
There's a lot of Utena crossovers that would be really great; I've always wanted to see a Ranma 1/2 crossover where Kodachi and Nanami meet. And there's all kinds of interesting stuff you could do with the Utena movie and things that are either in a totally different genre but similar visually or things that are technically in a similar genre but visually completely different. Even—maybe especially—in a written medium. So stuff like Sailor Moon/Utena would be pretty cool. Actually, the idea of 'student from thing x transfers to Ohtori Academy' is pretty winning all by itself.
But AS I'M SURE YOU, BECCA, ALREADY KNOW my favorite unwritten Utena crossover is the Gokusen one I promised to write you and then never finished because i was in a bad place emotionally to be writing humor at the time.* (It is still in the works, though! How could I not write such gems as 'Shin, grown-up law student, is constantly mistaken for a middle schooler' and 'Yankumi makes up elaborate plan that involves kicking Akio in the face'. How could I deprive myself of that.)
I just adore the idea of Yankumi putting together one of her complicated undercover disguise plans to root out bad stuff going on in a school, and the idea of her going after somebody like Akio is really satisfying. The idea of her vs. Akio specifically is GREAT, because if Akio is vulnerable to anything it is a) really sincere people who can't be seduced or distracted and b) physical attack.
There's also the aspect of Yankumi and Shin as a grown-up team running their own operations, where Shin is either in or just out of law school. They're on their own, without backup, in a very dangerous place—but because of their personalities and history, you'd never have to be too afraid for them. Yankumi is kind of uniquely placed to be able to deal with or avoid the particular pitfalls of Ohtori Gakuen, and while Shin is less so I do feel like the things that would be aimed at him are things he could deal with.
Except for being mistaken for a middle schooler when he's a graduate student disguised as a high schooler. he is not equipped for that.
*I still feel like I'm not quite ready to sit down and write it yet. It's hard for me to be whole-heartedly, genuinely funny—I find that weird dadaist humor that's popular on Tumblr right now to be much easier.
December meme post is here.