I would pay a great deal to see a well-written Howl/CLAMPverse crossover. Have you ever read the Chrestomanci novels, or The Homeward Bounders? Those two are also some of DWJ's best, and they would work really well with TRC or HOLiC.

Castle in the Air was terrifc, even though I would have liked more of Howl and Sophie. But it was still hilarious.