This idea is a combination of two separate ideas that eventually just sort of came together. The first was Obfuscating Stupidity I kind of like this trope and would like to see Ranma use it but the problem is that characters usually are like this as a result of guilt and trauma Vash, Kenshin, Urahara, Kyoshiro, Kurama. My other idea was Ranma becoming a doctor/healer though this was more due to the fact that I liked Trafalgar Law’s abilities which requires medical knowledge to properly use. It actually wasn’t that hard to duplicate once I started looking up the mechanics of the Ope Ope no Mi as its just a combination of dimensional manipulation and spatial teleportation.
The problem was that for along time all I had was these basic premises. The OS is not hard to believe, Ranma is pretty quick on the uptake when it comes to something he cares about and he in the manga at least Ranma does like to downplay his skill. Then I started thinking that a lot of those OS characters were doctors so the same thing that causes Ranma to go full OS is what causes his to want to become a healer as atonement. The Doctor thing is pretty easy to work, it is really easy to figure out a way to have him train, and even live with, Tofu-sensei for one option, even just the simple option of him needing a part time job and he offers, or maybe he got kicked out again, and Tofu lets him stay, and he helps out around the office to pay him back for letting him stay. Though he doesn’t really need Tofu for anything but the really advanced stuff as outside of training reading seems to be one of the things Ranma does the most.
I finally came up with having Ranma kill an innocent for the transition. Ranma is not a killer, that is not to say Ranma won't kill, he would but only as a last resort or if killing would save peoples lives. Putting him in a situation where he is forced to kill most likely against his will would get the ball rolling. I was finally able to connect all of the dots by crossing over Ranma with Psyren. Ranma could either be looking for another part time job or the dojo receives another call for help either way he receives the red card fills out the questionnaire and is transported to the other word. Like I said I originally thought up this idea because I liked Trafalgar Law’s ability. There is a PSI ability called Cure that essentially projects the psychics ability to heightens their body's natural healing properties onto another person. Apparently, it is a rare form of PSI that few people grasp the concept of. This seems to be a great match to have Ranma be a doctor.
There is also an Advanced PSI type called Zone. As opposed to Nova, which the user becomes an "existence" instead of staying as a "person" by taking control of the entire biosphere, in this type, the user takes control of any one field of the biosphere, either the atmosphere, lithosphere or the hydrosphere. Ranma would have to upgrade his medical knowledge to fully utilize cure and upgrade to his Psi ability could be the finished product of his studying and training. Ranma could get that ability instead of Melchsee's Door. The ability is useful to someone who is already a trained doctor, so Ranma would start out with the basic knowledge he learned from working with Tofu but to use his new ability to the fullest he would have to expand his knowledge of medicine and anatomy.
As for what would get Ranma to kill someone. There is a character from Gold Digger named Zero Kazama the villain Gothwrain put 12 Geis’ that constantly push in to destroy his essence, but the more he served Gothwrain the weaker they become. When he completes a labor a geis vanishes and after twelve he’s free. The commands are so strong that when one labor trapped him in some kind of And I Must Scream type situation for centuries to the point where he completely lost his sense of self he still completed it. this could be used to explain why Ranma would change so much. When Zero found out what happened to him he just gave in and tried to complete the labors as quickly as possible (sadly it took millennia and he was killed before he could complete his final one, but I can see Ranma fighting something like this tooth and nail unfortunately there will eventually be a point where he cant fight anymore and will succumb to the command. Luckily in Psyren the nature of the game means that he could save who he killed but he would still remember having done it.