Well, you didn't specify how many ideas we can postulate so here's a few that comes to mind....
My traditional wish is for a crossover/team-up with Hoppy the Marvel Bunny, of the Marvel Family. Especially if you use the light-hearted, fun tone of the Fawcett books of the '40s & '50s (which is well-suited to both titles and characters). The Rock of Eternity could be used as a gateway to Usagi's world.
Ooh, how about a crossover with the Thundercats? The various animal species abound (especially if you use the format and scenarios of the most recent cartoon, which was great). Maybe Jei will begin teaming up with Mumm-ra (both beings' powers associated with 'gods') but perhaps Usagi will have to eventually team up with Jei to stop Mumm-ra's latest plans because it conflicts with Jei's goals and vision. Just a rough sketch of an idea. (Plus, seeing Lion-o and Usagi fighting side-by-side would be cool).