What would I like to see in a hypothetical 6e?
--- The modularity of design* we were promised during 5e design that never appeared in the end.
* - by this I mean that the game isn't just one system but a whole lot of not-necessarily-similar subsystems, and that changes or kitbashes to one to one will not have much if any impact on the others.
--- Backwards compatibility with, and-or easy and clearly-explained conversion to-from, ALL previous editions, such that those still playing any previous edition can still benefit from (and thus be tempted to buy) the new material.
--- No built-in preconception of "this is the level where the game ends" - leave it open-ended, but with a disclaimer to the DM to the effect of "should your game get beyond about level [range x-y], you're largely on your own; though the tables and charts can largely be extrapolated to some extent". In other words, instead of saying the game goes from 1-20 or 1-30, just say it goes from 1-x where x can be whatever number you-as-DM feel comfortable with.
--- An end to jack-of-all-trades characters: no multiclassing, all characters have clear and unavoidable weaknesses as well as strengths
--- Flexibility enough to handle many different modes of play e.g. mixed-level parties, parties of 1 PC up to parties of 15 PCs, sandbox, hard-railroad, slow-pace and fast-pace, zero-to-hero, heroes from the start, etc.
--- Good stand-alone adventures - modules rather than hard-cover-book adventure paths, and lots of 'em - maybe even released before the core rules but at the very latest released side-along with them. And at least one example of each 'type' of adventure up front as well: a dungeon delve, a city adventure, a wilderness adventure, a sleuthing adventure, a war-front adventure, a courtly adventure, a maritime adventure, and so on; covering a reasonable range of levels right out of the gate. EDIT to add: and along with this, a how-to-design-adventures publication for DMs that draws on specific examples from these adventures in terms of how element-x and design-y are likely to affect play.
--- Emphasis on player-side simplicity, let the DM worry about the complex under-the-hood stuff.
I could go on for ages, but that's probably enough for now.
