Prince of Tennis X Vampire Knight
( I think it would be funny to see Seigaku and maybe some members of other schools attend Cross Academy. I prefer the plot centered around Ryoma and not having him really weak compared to the vampires. I just think it would be funny because Ryoma's personality would clash with the night class.)
Pairing wise, yaoi would be best:
Ryoma X Kaname
Ryoma X Zero
Ryoma X Kain
Ryoma X Shinri
Ryoma X Takuma
Ryoma X Aido
Ryoma X Fuji
( Love triangle would be awesome too, or having Ryoma paired with more than one or multiple people)

Prince of Tennis X Kuroko no Basket
Tennis teams and basket ball teams meet. Hilarity ensues. Ryoma clashes with Akashi and continually defies him and pisses him off. Ryoma is very blunt so I can see the first words he says to Kise being, "You're annoying."  To which Kise cries anime style. Ryoma would probably clash most with Kagami, Akashi, Midorima, and Aomine. Which would be entertaining. The gathering of the sadists: Fuji, Yukimara, and Akashi.
Ryoma is able to notice Kuroko which is what first draws attention to him.
Pairing: yaoi probably. No Sakuno X Ryoma please. She's annoying.

Prince of Tennis X Bleach
No one dies but the tennis players are able to see soul reapers. Obviously I still prefer it centered on Ryoma. Maybe his father was a soul reaper? Or he gains a power like Orihime? I can see him pissing of Aizen and Grimmjow which I would find entertaining. Tennis players meet soul reapers and hilarity once again ensues.
Pairings: yaoi?
Hitsugiya X Ryoma
Others?

Prince of Tennis X Final Fantasy
Ryoma ends up get transported to Shinra somehow and causes trouble. Draws to attention of Sephiroth, Genesis, Angeal, Zack, Vincent, and/or Rufus. Sephiroth didn't go insane, Genesis and Angeal didn't die, and Cloud doesn't exist(maybe)
Pairings: yaoi:
Ryoma with ones listed above.
Ryoma X Tifa
Ryoma X Yuffie

Prince of Tennis X Magi
Ryoma gets transported to the Magi world somehow and starts to cause trouble. Maybe he could be a fifth magi? He meets Sinbad first and it's an interesting meeting(Sinbad gets ignored) Sinbad becomes interested in Ryoma and decides to pursue him and so does Judal.
Pairings:
Sinbad X Ryoma
Judal X Ryoma