DY (nice to talk to you too),
I didn’t mean to say that after Ravina and Rav Ashi there could be no more new ideas and applications of halacha. I meant that nothing post Ravina and Rav Ashi can be a horaah that binds Klal Yisroel. If Rabbeinu Gershom makes a takanah it will only bind the population of the geographic area where Rabbeinu Gershom is the accepted authority. If the Rambam paskens one way in an ambiguous Gemara his psak was not binding outside of Egypt (or wherever he was the accepted authority).
Similarly we cannot make new Gezeiros in the post-Talmudic period.