Here's a rather crazy idea for a Ranma 1/2/Persona series crossover:
Some months or so after the events of the Persona 4 Arena duology (those two games are set roughly within a couple of days or so of each other, for the most part), Aigis is sent on a mission into the Chinese hinterlands to investigate some Shadow readings there. While going through the Jusenkyo area, she inadvertently falls into one of the springs there. Fortunately, it's one of the previously uncursed springs; even more fortunately, she's able to pull herself out. However, she spent enough time in the spring for it to become cursed.
Several months later, Ranma Saotome and his father Genma make their fateful training visit to Jusenkyo. Ranma gets knocked into what turns out to be the same spring that Aigis had fallen into, so now whenever he gets splashed with cold water, he becomes a robotic girl who can wield Personas. Keep in mind that the original Aigis is still running around.
Now imagine the sheer chaos that this could bring to Nerima...
Some months or so after the events of the Persona 4 Arena duology (those two games are set roughly within a couple of days or so of each other, for the most part), Aigis is sent on a mission into the Chinese hinterlands to investigate some Shadow readings there. While going through the Jusenkyo area, she inadvertently falls into one of the springs there. Fortunately, it's one of the previously uncursed springs; even more fortunately, she's able to pull herself out. However, she spent enough time in the spring for it to become cursed.
Several months later, Ranma Saotome and his father Genma make their fateful training visit to Jusenkyo. Ranma gets knocked into what turns out to be the same spring that Aigis had fallen into, so now whenever he gets splashed with cold water, he becomes a robotic girl who can wield Personas. Keep in mind that the original Aigis is still running around.
Now imagine the sheer chaos that this could bring to Nerima...