Peacemaker Kurogane and Samurai X (Being Rurouni Kenshin's OAV). The two series take place at the exact time (The Bakumatsu Revolution) and focus around the exact same events (like the Ikedaya Inn Incident) The crossover potential is amazing because you have the two opposing sides (Shinsengumi and Choushu).
Other than that, I can't really think of any crossovers I'd like to see. Tsubasa's a pretty good series though.