Three that I can think of. In one I can't remember the name of Kenshin replaces Ichigo, the other two are:
Clean and White
The ancient sword-style deals in death for a reason.... Crossover with Bleach, prequel to Mending Songs.
Mending Songs
In 1895, a shinigami relearns the world of the living. Bleach crossover
EDIT
Fosfor's Bleach/HP cross Frozen Ring has Kenshin as 10th Division 3rd seat.
