I don't really want to be too specific about this topic... and I may actually kill it with the first post.
Valis II was the earliest Valis entry on PCE for years... until a port of Valis I was released for PCE (years later). Valis I appeared on numerous platforms (Famicom, PC's) but folks outside of Japan only had access to Valis I on Genesis. So, TG-CD owners would have to look to Genesis for the first installment f the Valis saga.
Rayxanmber II is the earliest PCE entry in the series... the first game was on FM Marty?
Time Cruise II was originally just Time Cruise, but it evolved and changed over the years. I think it was a wise move because a "sequel" implies a successful first entry... and it gives a fresh veneer to a project in development for years (?).
Strip Fighter II was obviously riding on the success of Street Fighter II.
Final Zone II was a sequel to X68000 and MD/Genesis FZ I. FZII is unique because PCE/TG-CD was the only platform it appeared on? Correct?
So, not that anyone cares, but I find this stuff interesting. Am I forgetting anything?
Feel free to expand the scope of the original topic in creative ways (like, PCE games that have sequels on other platforms... like Cho Aniki?)