Go ahead. Let it out. What do you REALLY want from a 6E that you know you aren't going to get?
I'm actually pretty happy with the core of 5e, so I'd mostly be looking for small tweaks to that (but a lot of them). But where I think the game really needs an update is in the DMG and the MM.
For the MM, I'd mostly want to see the various monster tables taken out of the appendix in the DMG and reinserted here. I'd also want the "Monster Features" table from p280 of the DMG moved to the MM (and future monster books should include a similar table as standard).
The big thing, though, is that the monsters themselves need a thorough overhaul - too many of them are just big bags of hit points. And they should bring back the Minion/Elite/Solo split and the explicit monster roles from 4e. Oh, and get rid of all the fiddly little CRs, and instead give every monster a level range that they're considered appropriate against - so a goblin might be a "tier 1 minion" or whatever.
The DMG I would suggest needs a complete rewrite, and should be squarely aimed at DMs of middling ability - newbie DMs should be pointed to the Starter Set and/or tutorial videos on the website, while experienced DMs don't need the help so much. But for mid-level DMs they should aim to provide tools to reliable craft decent encounters/adventures/campaigns/settings in various styles.
So I'd start at the lowest level there: the encounter. This one basically follows the 4e model - an encounter has a number of slots, which are then filled with a set of monsters of an appropriate level. Add advice to include a mix of monster roles, on the use of minions, solos, etc, and adding terrain and situational effects, and you have something workable.
For adventures, I would suggest the system should build these in a similar way to PC construction - where PCs are built by making some big decisions (race, class, background) and then some customisations (skills, equipment), so adventures would have a setting (dungeon, urban, wilderness, planes), a types (treasure hunt, mystery, escape), and a level range.
For each of the settings you then have a bunch of sub-settings (so wilderness might be forest, or mountains, or jungle, or whatever). For each of
these I would present a 2 or 4 page spread gathering together some dedicated rules, suggested terrain features, suggested monsters (doubling as a random encounter table), and so on.
That way, a mystery set on a river boat ("Death of the Nile") has some key similarities but also key differences to an escape set in a sinking ship ("The Poseidon Adventure"), and also some
different key similarities and differences to a mystery set in an isolated mansion ("And Then There Were None").
Oh, and don't forget to include
loads of example traps, plot complications, escalations, twists, and so on and so forth. These are basically equivalent to spells that the DM uses at various times, so they demand a significant page count be dedicated to them.
The repeat much the same structure for campaigns and settings.
(I would then finish up with a fairly long chapter on high-level adventures, on the grounds that this may well be the
only support those will ever get, and then retain the DM's Workshop more or less as it is. Random treasure and the magic item descriptions would be moved to a very long appendix.)