There is a reason that this isn't a good idea in a continuation fanfic. You are dealing with characters that have tons of canon and fanon behind them, then start out the story 2 years after with what amounts to a massively OOC main character. You can say that you'll do flashbacks to develope the character, but it will bug everyone until that is done and many will still call BS coughcough.
Most glaring plot-hole so far, simple, the fiancees, rivals, parents, and enemies would have found him almost immediately if he or she had ended up with amnesia. They wouldn't have put Ranma into something like witness relocation and would have listed her and/or him as a Jane and/or John Doe and matched up with a Missing Persons report. Mousse and Ryouga probably couldn't pull off a good enough scam to cover-up Ranma's disappearance, the others would have still come looking.