Oh this is a really good question! I’ve thought about this a lot in my spare time, and pretty much any crossover is a good crossover, but off the top of my head I think the people I most want the TRC family to meet would be people like Tomoyo (which I’m 100% sure actually happens anyway, so this is good with me) and other people from CCS (though we already have Toya and Yukito, so I’m actually pretty satisfied on that front).
I would love for a few more X/1999 characters to show up, but preferably in non-heartbreaking ways? For example Subaru is precious and I feel like he would have some pretty good AU options, but if that also comes with Seishirou showing up and extended suffering on Subaru’s behalf then I would just feel bad all over again. I think I would really love to see the Magic Knight Rayearth girls again, but with both Mokona and the Kudan thing so prominent at the start of the plot I feel it would be quite unlikely. I would also LOVE to see Kero or Yue or any of the cards from CCS, but again, I suspect that they might be world specific, so I’m not really hopeful for that option.
I’m kind of interested in whether or not characters that die in one universe are considered dead in every universe. Like we know for a fact that Clow Reed is dead in both CCS and in TRC!Sakura’s homeworld, but does that every incarnation of Clow is dead? Do they share the same fate, or are there still versions of Clow out there screwing with everything and being a troll in general? That’d be interesting to see. Alternate versions of Yuko might be even more fun, now that I think about it.
Actually if any of the TRC family ran into alternate versions of themselves I would be ridiculously happy. I absolutely want to explore things like how growing up in different worlds makes them different, but also how they’re essentially the same person at their core. There are so many options there. What if they ran into AU!Not-a-ninja-at-all Kurogane, and how would TRC!Kurogane react? Or an AU!Fai that was completely serious and never told any jokes? Or AU!Syaoran and Sakura that were totally together and would that trigger anything in Sakura’s memory? Would Syaoran say anything? Oh god he would be miserable. I kind of want to see it. Oh, or if they ran into any AU!versions of themselves that were EVIL?
THERE ARE SO MANY POSSIBILITIES. JUST GIVE ME ALL OF THEM.