Surprisingly, I haven't read any of this kind of xover. It seems to be a perfect fit for the two. What I see happening is that Ranma met all the sensei of Ryozanpaku at some point of this travel with Genma and each have left some kind of impression on him. Of course, during all of these time, Genma was driven away and came back to steal him after about 3-4 wks of training or lesson. I would also like to have Ranma meet Miu. It would be after she got her cat pin and Ranma phobia. I think it would fit will if Ranma was in cat mode at the time and met Miu. She would be playing at a park when Ranma jumps her from behind, and Miu tries to throw him. He knocks her down instead and starts licking her face because she had ice cream smears around the mouth. She takes him home as a pet and of course the Elder recognize the symptom right away. He plans to help Ranma overcomes it, but before the process could be completed, Genma steals him away. Ranma and Miu forms a strong friendship as they are both talented martial artists. He promise her that they would be best friends and would be the best martial artist ever, even greater than the elder. Unfortunately, when Genma stole Ranma way at night before completing the healing process, Ranma was in the middle of meditation to overcome the fear. This cause some mental scarring and lost of memory.
Flash forward. Depending on when in the timeline you want to be. If it's the neko ken episode, then Ranma see flash, like licking Akane, he saw a flash of the time he did the same thing to Miu. These memory would be brief so he wouldn't understand them.
If we go to after the Hiryuu shoten ha or moko takabisha, then the Yami would try to recruit Ranma or Genma owes them some debt.
Hmmm..I just thought of another angle. Maybe Ranma was sold to Yami and by some miracle, Genma got him back. it's also why they were always traveling and never staying any where long. Genma thinking that Yami had forgotten about them, returns to Japan. If this was used, then Ranma has a dark side that he keeps in control at all times through is confidence. Instead of Genma stealing Ranma back, Kensei was experimenting with trying to force the release of the cat berserker and ranma escaping instead. Maybe the Akira Hongo got his scar from the berserker rage. After the escape, Ranma would finally runs into Miu (see above).
I guess you could also do the same thing for any of the sensei. I think Hayato would be a good one to be sold to.
Now, getting Ranma to return to Ryozanpaku. I guess there could be any number of ways depending on when in the timeline for each of story. For me, I would set early in the story for Kenichi, somewhere before D of D tournament. Ranma would be in the same year as Miu, but at least 6 month older. Ranma would meet Miu again and Kenichi sees them in the background. However, he sees them fighting and thinks Ranma is a member of YOMI trying to capture Miu. Kenich jumps in the attack Ranma and only to find that it was like attacking wind, very much like sparring with Miu. Ranma finds Kenichi to be an imteresting fighter and a change of pace from his regular opponents. Unfortunately for Kenichi, Ranma has a lot of experience fighting and also a prodigy so he wins the match, though it took sometime due to Kenichi's endurance. For Ranma, it was liking fighting Ryouga without the hot temper. It was dark when Kenichi awakens and went to look for Miu and found her sitting on the porch. As he approaches her, he noticed that she wasn't alone as he had thought and finds out it was Ranma. He would also notice that Miu is comfortable and happy around Ranma. Feeling rejected, he returns to his room and swore to challenge Ranma again after training some more.
Kenichi awoke to the sound off fighting and thinks they're under attack and runs out. There, he got another shock when he saw Ranma sparring with Miu. For a moment he thought Ranma was trying to hurt her and tries to enter the fight, but got trip up by Akisame. He tries to protest, but was told to shut up and watch the match. Confused, he studied the fight and finds that Miu was fighting all out and Ranma keeping up without a problem. Since both were arial combatant, the match were usually in the air.
It's all I have time to write at the moment.