I think that 6e would be an iteration of 5.5, which is to say an iteration of an iteration. It would look different enough from 5e that it likely wouldn't be super backwards compatible but I think a few core pillars would remain. In my mind, there are two potential futures:
Future A, it's feats on feats and prestige classes; basically the 3.5 of 5.X and will see some success where they try to tone it down in another "rules revision" where they'll find themselves being better off returning to 5e.
Future B is the timeline where it's a generic system that's a cross between GURPS 4e and WFRP 4e and will be marketed as "we provide the rules and theme, you create the flavor". Personally, I think that would be a bad approach for them. Partly because at that point it'll be Mythcraft but made by WOTC and partly because they'll kill alot of specific stuff they could make. It would be a significant departure I think from their current design philosophy. I also think that that sort of design approach would leave a lot of homebrewers feeling a little.... directionless, which will kill a lot of chatter and 3pp innovation around the game.
Do I think that a 6e is innevitible? probably, not because I think they need to change the system but I think at a certain point they'll have it done for hype and shareholders in the company or something. When 6e does arrive, I probably won't be playing it and will sound like my dad talking about the good ol days of 3.5 but instead I'm talking about 5e. I'll have other systems I'll be playing- and at that point I'll be making 5e content calling out to the people like me who do the same thing.