Just finished reading the manga 'Hakodate Youjin Buraichou Himegami'. A very good series for a Ranma crossover I think.
http://www.mangaupdates.com/series.html?id=16568
Great action, cool powers, and beautiful girls. Plot device: Nanban mirror.
EDIT:
Well, sending Ranma to the past is one of the two ideas I had. Here's the other one.
The descendants of the bad guys come back to Japan to try once again to get their hands on the power of the Himegami.
Ranma could be either the new seal or a descendant of the himegami who inherits the power to fight off the youjin.