What I'd like (and probably won't get, but whatever) in 6e:
--- species mechanics strengthened and expanded: more baked-in benefits and penalties for different species, along with...
--- ...way fewer PC-playable species in core; an optional splatbook can cover all the oddball species
--- greatly downplay (ideally to the point of near-eliminate) the "character build" side of the game; make char-gen fast and easy and let the character develop from there through what it does in play
--- along with this, make characters easier to play - e.g. fewer feats-abilities-bonuses-etc. to remember - such that playing more than one at a time becomes a viable option
--- more niche protection for all classes; if this requires reducing the number of classes to avoid overlap, that's fine too
--- a flat power curve such that low-level PCs and monsters can be and remain at least a minor threat to their higher-level counterparts over a greater range of levels; this also allows mixed-level parties to function
--- drop the 'tier' concept and have each level's power increase be (on average) roughly the same throughout
--- it's a zero-to-hero game, focus on that journey rather than the destination
--- ignore cries for more power with every level; either make some levels "dead" (no new feats, powers, or abilities) or reduce the number of levels overall, such that high level characters still fit in with the setting and aren't demigods
--- design to 20th level open-ended but make it abundantly clear with great big letters that the usefully-playable range is only intended to go to about 12th and after that you're on your own
--- release an optional splatbook later that takes care of beyond-high-level play
--- greatly slow down level advancement; focus on the day-to-day play and emerging story rather than on "moar numberz"
--- no appreciable power gap at the very low end: commoner, regular militia, and 1st-level character should be on a smooth -1st, 0th, 1st power progression that fits in with subsequent levels
--- design the core game to be very harsh and unforgiving on its characters then include easy-to-implement DM-side options to make the game a bit kinder
--- more randomness in character generation: rolled stats, rolled hit points, etc.; along with an underlying philosophy of "play what the dice give you" rather than players coming in with preconceived character ideas
--- a more easy-come easy-go design and "sense" in terms of wealth, items, maybe even levels, maybe even characters
--- make downtime the fourth pillar and lean into it; forcing downtime is easy via making training a requirement to level up, and downtime is when the PCs get to engage with the non-adventuring parts of the setting
--- starting right at core release and continuing for years thereafter, a long run of single standalone adventure modules of all levels that showcase the system and yet are also good adventures on their own. No more hardcover campaign-in-a-book APs.