My idea is similar to what has already been said: No Golden route.
Though I liked Fates and the three routes it had, I thought Revelations significantly undermined the other two.
That and judging by this thread alone, a Golden route can only be a recipe for disaster. Let's not make TH a bigger mess than it already is.
Personally, I would much rather have a 5th route not a side story. One where Byleth decides to part with Jeralt and strikes out on their own as a free roaming mercenary rather than going along to Garrech mach. Much more potential there. Would love to see how the story changes with Byleth never becoming a professor.