Ranma's f!twin swordswoman and konoichi oc
Reina x Kasumi Tendo x whoever or not
Nemuri aka Ranma's story location is the centerpiece for the conversion/hotspot.
Reina and Ranma have a twin bond so they can talk and scense each other.
Reina can have a system but it has to fit in with the them so a supernatural hunter or a martial arts system revolving around her swordswoman and konoichi arts think Shigure Kōsaka from Kenichi strongest disciple.
Reina can be cursed by the Jusenkyo/ cursed springs but not as anything male(yuri book) make her a Kitsune (9 tailed fox spirit) or another yokai like a Yuki-onna(snow woman).
Nodoka Ranma's mom will obviously raise Reina (we all know how his dad is though his mom's not much better really). Ranma's life is generally the same he will be more educated and grounded from his bond with Reina who while not going to regular school was technically homeschooled while on her own training journey with Nodoka. This journey is where Reina runs into the Supernatural aspect it's doesn't really need to be fleshed out unless you want to but it can be brought up and mixed in with the Ranma and Sailor moon story lines.