The question of pet companions aside (not a big fan of the current implementation - if anything, just allow everyone to have a loyal dog companion

- but that's personal preference), there is a number of things I would like to see in a potential sixth edition.
However, I also have little hope for them to actually appear (also, I agree that 6e is not happening until the sales decline - the may do something in 2024 for the 50th anniversary of D&D, but that might as well be just a leather tome version of the basic books).
Still, since this is my chance, here's my thoughts:
Product policy:
- I would like to see a return of proper campaign setting books (like the 2e boxes or the 3e Forgotten Realms Campaign guide) and also smaller books/"regional guides" with details. Not everything needs to be there in excessive detail (probably small soft cover books of ~100 pages would be enough), but I really would like to see modernized (and timeline-adapted) versions of books/boxes like "Waterdeep and the North", "Empires of the Sands", "Menzoberranzan", etc.
- Like someone a few pages back, I would like to see more independent adventures, not only adventure path-style campaign books.
- I really would like to have official (searchable and indexed) PDFs again like we have for 3/3.5e
Layout&Structure:
- The books would really benefit from a better index and, while were at it, also better layout that highlights rules parts more clearly (also, don't hide rule parts in plain text)
- As someone already noticed, a better index would also be much appreciated
Translation policy:
- Probably not relevant for people that only buy the English books, but for the love of god, the current nonsense of putting brand management before everything else and not translating so many words should stop. Iconic names like Waterdeep, Evermeet, etc. were translated in Germany for every edition since the first, and translators were allowed to come up with names that at least felt like an adequate use of the German language. The fact that we now have titles like "Waterdeep: Drachenraub" is one of my major dissatisfactions with 5e.
Abilities/Skills/Checks:
- On the one hand, I like the simplicity of advantage/disadvantage, but a) it is too coarse-grained and b) there is an awful lot of rules that dictate when you should get advantage. I would very much prefer a system that leans more into DM judgement, but allows at least one, maybe two more levels for the relative fictional position. Theoretically this could be done by introducing major advantage/disadvantage and rolling 3d20. Beyond three dice, though, this might start to be too slow at the table (and the probability distribution shifts a little too much towards the higher end of the spectrum)
- I would like a more fine-grained skill system - that's probably the only thing I really like about PF2. In 5e, characters of the same subclass often fill rather "samey", and I really would like to have the option for a little more customization here.
- Related to the previous point: I am not a big fan of the current progression system where you become better just by levelling up. I would rather like to see skill increases for that.
- I would like to also see skills for combat and magic (probably an even bigger step for D&D than some other stuff in my list)
- Go back to roll under attribute (probably: +skill) or drop the 3d6 / 3-18 attribute scores. If we are just using the bonus, then we can also skip the old stuff that is now devoid of mechanical meaning.
- Split Wisdom into Willpower and Perception
- Split Dexterity into Agility/Speed and Nimbleness; use agility to determine movement rate
Character Classes:
- Generally I feel, there is a lot of potential to weed out classes here and sharpen the concept for the remaining ones, e.g.
- Sorcerers and warlocks could be merged - you either go the long and hard way to learn magic (Wizard) or you bargain with a power of questionable morality or alien motives to gain power quicker.
- Druids stop being full casters and are now only shapeshifting nature-lovers
- While we're at it: current clerics can go away - healing works much better in 5e without constant cure XXX spell spamming, and if you want to play a religious warrior, you can play a Paladin. Maybe introduce a new Priest class for religious scholars that can exorcize evil things and maybe also commune with their god for divine intervention - but that should be a new subsystem that does not work like wizardly magic
- Generally: no one gets access to the full spell list all the time (too much time wasted picking the right spells for the next day) - you learn a number of spells (receive it from your patron) and choose only from those
- Rogues are in a weird space right now where they feel like DEX-based warriors - I don't have a good idea on what to do with them right now
- On the other hand: rename Monk into Martial Artist/Ninja, actual monks like Little John could be a subclass of the Priest class
- Rangers become lightly armoured scout types - no one gets an animal companion (see initial comment)
- Multi-classing could go away completely - rather allow people to adopt additional (mechanically represented) traits with feats or something like that
- Currently a lot of the "powers" characters gain through level progression are combat-related - replace them with more options that support the other pillars of play
Hit Points/Combat:
- Limit hit point inflation or come up with an alternative system, where hit points only represent level of exhaustion and will to fight, and there is a (low) number of actual wounds characters can take.
- Probably critical hits should always deal actual wounds then.
- Actual wounds shouldn't heal without treatment on rests
- Encourage tactical play through situational bonuses/penalties (see more fine-grained advantage/disadvantage system); also: add more examples on what people could do in combat and maybe also offer advice to DMs on how to adjudicate the respective difficulties
- Introduce some sort of success level for attacks that carries over to damage
- Consider modelling combat/conflict as opposed rolls instead of flat AC/stat-based checks; armour then does damage reduction instead of making you harder to hit (this is, admittedly, a pretty major change); maybe only allow defence against ranged attacks if you have a shield
Magic Items:
- Allow special fighting techniques, fancy rune tattoos or something like that as alternative to magic items
Races/Backgrounds:
- Consider tying ability score increases to backgrounds instead of sub-races; maybe consider also moving race ASIs there (I have become so used to them, though, that I don't mind the either - but making an elfen black smith a little stronger than a dwarven scholar seems only logical to me)
- Following a sentiment uttered before: Halflings go away and are merged into Gnomes.
General Tone/Settings:
- Having started with 2e, the plethora of player races in 5e does not really match my mental image of the D&D fantasy world - this is, of course, highly subjective, and people seem to love their Tieflings, Dragonborn, Aasimar, etc., but I would really like to see a variant of the game with a more classical feel.
- Generally: I feel, there is room for a few campaign settings that branch off the main line and offer a different perspective.
Tiers of Play:
- For the most part of my D&D history, my characters never went much beyond level 12 (I think, I have a handful of characters that go up to 14), and the current adventure modules also seem to live in that range. So maybe consider two lines, where "basic" 6e goes up to level 12, and "advanced" 6e goes up to higher levels (maybe even beyond 20)
I could come up with a few more things, but I really think I have written enough by now. Sorry for the wall of text, guys