I doubt we will see 6e within the next ten years, tbh.
But, if I had Mike's job, my priorities for a 6e would be:
1. Backward compatibility, and don't call it 6e.
2. Do not break apart the ranger, or rebuild it to use terrain as it's subclass, or any such conceptual overhaul. The ranger needs to do what it already does, but actually be effective at all levels, and for every choice to actually meaningfully impact the character.
[sblock] Natural Explorer should still include a favored terrain choice, which should give benefits to exploration in general, but particularly in stuff like movement, stealth, surviving hazards, etc, so that it is sometimes useful in combat as well. It should also have always on travel benefits for the whole team. If they can be tailored to a terrain but still useful anywhere, great.
Favored Enemy should stop being about choosing a species or similar grouping, and instead be things like "behemoths, hordes, outsiders, conjurations, spellcasters," etc. Dragons might be fine, but if things like Rocs and Griffons can be put in a group with them, that's better. Then, the benefit is to tracking, interacting with, evading, and fighting those types of creatures. Things like "when a flying creature that is larger than you leaves your threatened space, you can make an Opportunity Attack even if they Disengage or otherwise have an ability to move without provoking them." or the Hunter Ranger's Horde Breaker benefit. Basically consume the Hunter into the favored enemy class feature.
Also, Rangers would have known spells equal to their Wisdom mod+half level, and Favored enemy would give the ability to use Hunter's Mark without a spell slot against a creature that qualifies as your favored enemy, or something like that. I prefer to keep HM a spell, though. I wouldn't replace it with a class feature, as such. [/sblock]
3. Give monsters skills, and bring back thinks like "leader" and "elite" classifications as templates to add to monsters, also clean up the math of monsters so that they are easier to adjust, and experiment in playtesting with different ways to make monsters more dynamic without making them overwhelming.
4. Move further away from the idea that humanoid races can be born evil.
5. Experiment with giving Fighters Legendary Actions instead of more extra attacks.
6. Make Inspiration more front and center, in part by having characters automatically have inspiration when they gain a level, or maybe even at the start of an adventuring day, or something.
7. Experiment with Extended Rests, as an expansion on Downtime. Provide alternate rules for only recovering some of your resources with a Long Rest. Perhaps half your spell levels in slots, for instance. Basically model something similar to the Journey rules in The One Ring and Adventures in Middle Earth, where you have to rest in a place of true safety to fully get back to 100%. But also provide bonuses when you take an Extended Rest, especially if you do so in a place where you are very well regarded, or in your own stronghold, etc. See 6; maybe you gain Inspiration at the end of an Extended Rest, in addition to being able to gain it by doing cool stuff and playing your character well.
8. Make Travel more interesting. This would be a big goal, but I don't have any concrete ideas for it off the top of my head.
9. Include actual advice for crafting, running a business, leading an organization, etc. Doesn't need to be whole subsystems, but some degree of support for DMs that don't want to invent a whole system from scratch or spend the whole campaign playing "idk just...um...roll a thing. okay, that seems pretty good...XYZ happens".
