Top level goal: Grow the hobby by retaining existing customers and welcoming new ones.
Breakout: It must be recognizably D&D to the vast majority of existing players, and new players can jump into a game with their own level 1 character with minimal reading and time invested.
6E goals to help with this:
Power Sources
Bring back the concept of Power Sources, and affiliate them with ability scores (or multiple ability scores).
Ability scores
An issue both existing and new players have, for different reasons, is how some ability scores are vital and some aren't. For veteran players, there are concepts that just are sub optimal. A high INT tactical fighter should be reasonable, but the mechanics don't reward the INT and since it comes at an opportunity cost of other ability scores it leads to poor characters. For new players the ability score to class rubric hasn't been burnt into their brains, and their concept may be a high STR, low WIS druid. That won't serve them well at the table alongside more conventionally built characters.
There are a bunch of ways to improve this.
- Balancing ability scores so they all have flavorful things that will add to all characters.
- Allowing classes to be based off various ability scores might be interesting. Is your bard an INT-based loremaster or a CHR-based performer?
- Make chassis classes like "unarmored caster", "warrior/caster", "skirmish/invoker", "caster/invoker" that then pick a power source, which determines the ability scores for their magical aspect. ("Invoker" was a shorthand for a more warlock invocation type.)
Race/Class matchups
Because of ability scores mods from races, and needed ability scores for classes (though lessened from the first issue), take a page from 13th Age (it's d20 OGL) and have the races give out a +2 from a selection of race-appropriate abilities, and class give +2 from a selection of class-appropriate abilities, and they can't be the same. Heck, have background do the same - I'd love to see it become more of an integral part of a character.
Powerful Races
The 5e races don't have a lot that impacts mechanically (outside of ability scores). Make race choice more meaningful by having more features, including some that don't come online until later levels or activated by feat.
This increased design space will allow more powerful races then currently allowed, such as large races and flying races - and being so will have a real opportunity cost in not picking one of the other races.
Multiclassing
I'm a fan of 5e multiclassing, but a lot don't. It also is really easy for a new player to shoot themselves in the foot. So take away multiclassing and replacing it with feats that allow substitution of features from another class (so a "mostly one with a bit of another') as well as some hybrid classes (much like the paladin is now).
Will classes allowing Power Sources, you can have the skirmisher/caster picking Arcane for a INT based arcane trickster type goodness, or Primal (druid) for WIS based ranger-like goodness, etc.
Since there is no more need to caster levels to be able to add in some ways like in 5e (sadness - I like that design space), you can also step away from the "spellification" of classes and give back special abilities to classes without making them spells.
Customization
Existing players want customization, new player need a low-barrier-to-entry which does not include reading through a lot of customization options and making irrevocable choices based on theoretical knowledge.
A solution is to add in a lot of customization later in play, with 1st level giving strong foundation instead. (No more subclasses that grant foundational things at 3rd level.) What customization is granted at 1st should have options that get retrained. (Not saying everything can be retrained, but perhaps 1st level options can be trained when you gain 2nd or 3rd.)
So a new player picks race (who you were born as), background (who were were raised as) and class + Power Source (who you are now), plus arranges ability scores and picks skills.
Customization comes in for everyone at 2nd and onward, and at a "character level" point of view, not just for your class.
Recovery
Better balance between at-will and other resource recovery/management models, with more knobs for the DM to tweak depending on their game style.
Keep
Concentration, Upcasting, Bounded Accuracy