I seem to be on a role with all these new plot bunnies recently. Too bad nobody seems to care, but maybe this particular combo will get someone's attention.
The timestamp for this particular fanfic is western Tokyo, several months after the defeat of Saffron and the failed wedding. Ranma, feeling fed up with the situation regarding himself and the fiancee brigade, has done the only thing a sane rational person would do in this situation and taken an extended training trip into the mountains on the western edge of the Tokyo district. What he finds when he gets out there, however, is not what he was expecting; instead of solitude, he finds a strange city tucked away in the hills.
What Ranma has stumbled on is the one city in Japan that is not controlled by the Japanese government; Academy City, a high-tech 30-years-ahead-of-its-time metropolis dedicated to the study of psychic powers, built on international territory, funded by governments and corporations the world over, and home to the events of the anime/manga/light novel series To Aru Majutsu no Index. With no particular destination in mind, Ranma decides this weirdly futuristic city is as good a place as any to get some training done. Unfortunately, local security doesn't agree with him; Academy City is extremely private, and without a permit nobody's allowed inside. Naturally, this doesn't bother Ranma too much; he jumps the security wall and vanishes into the city.
From here there's a huge number of ways the plot could go depending on where exactly in the combined Index/Railgun timeline that Ranma comes in, but the one I'm most interested in is the Sisters arc and the battle with Accelerator. Ideally Ranma will cross paths with Mikoto at some point and hit it off with her, though probably after pulling a Touma and getting a railgun in the backside or something. Psychic powers should be interesting for Ranma to deal with but I doubt he'll want to try developing his own once he hears about how exactly the Power Curriculum Program works. How exactly this all happens isn't tremendously important, what is important is that Ranma develop a close enough bond with Mikoto to want to stick his neck out for her.
When the battle with Accelerator comes around, I expect Ranma to hear something about the Level 6 Esper project and make some comment about how the scientists in this city are assholes, which then prompts Mikoto to elaborate on some of the other stuff she's heard or been directly involved in, which makes Ranma all the more certain that someone needs to do something about these people. Ultimately, Ranma fights Accelerator and nearly gets his ass handed to him, but ultimately manages to pull through. This, naturally, catches the attention of the scientists behind the Level 6 Esper project and Aleister Crowley, who takes great interest in this superhuman fighter who uses neither magic nor esper powers. Unfortunately for Crowley, with Accelerator dealt with there's nothing distracting Ranma from taking great interest in him and introducing a little chaos into Crowley's carefully-laid plans.