When I look at the numbers of books and options for 3e and 4e before a new edition came out....yeah not seeing that for 5e at the moment.
But if we are going a full new edition, it needs big changes...otherwise its not a new edition. Wotc may not go that route, but assuming they do...here is my speculation.
1) A relook at monster design. 4e did it right here, and 5e was a step back imo. The monster "types" in 4e really helped create interesting, dynamic challenges for your party to face. The monsters in 3e and pathfinder create more challenge right out of the box. 6e should return to some of that, but with its own improvements to design.
2) A true attempt to balance the game independent of encounters per day. There is a WIDE variance in how people play the game encounter wise, and its time we nipped the issue in the bud. I personally think short rest (aka encounter powers)....should just be per encounter. You get them every encounter...until you don't. Instead of "don't refresh until you rest" instead make it "get it back unless you can't". Give the DM leverage to say that a hard encounter or a special condition leaves the party "fatigued" and unable to recover. This allows for the draining of resources that is appropriate to certain stories, but not make it so core that we have to throw tons of encounters at a group to achieve actual balance. Aka refreshing your power is the default (which players tend to enjoy), and not refreshing is the special exception for "really hard encounters".
3) Another look at spellcasting. I still think they went too far with Concentration, its too overbearing, its too fiddly, there's too much tracking, and my players loathe it. I am all for reducing the spellcaster dominance at higher levels...but we can do better.
I also would love an attempt at more "automatic conditions". Lets delay the saving throws a bit, give the casters more automatic but weaker effects to allow the combat to go longer. Example, Dominate Person automatically charms you until the end of your turn, then you get a save. Pass the save and shake it off (which is how its often cinematically done in movies and books), or fail the save and succumb to full domination. Hold Person - automatically reduce your speed by 10, save to go full paralysis or shake it off at the end of your turn, etc etc. Casters get to enjoy more regular application of their spells...but the spells are not as big right off the bat...which gives the fight a little time to breathe instead of "okay monsters are all hit with the whammy, and 2 rounds later everything is dead".
4) Poisons: Poisons just never seem to feel right in any edition I've played. They are either ridiculously expensive for what they are, or the most powerful substance you can give someone. 5e has gotten a good bit closer, poisons are more reasonable than in 4e and definitely 3e, but we aren't there yet.
5) Stealth: I like 5e's approach to give things back to the DM, but sometimes something is so powerful, so difficult to understand, and so common in the game it really does need a bulletproof set of rules. Stealth is one of those. Its a mess in 5e, its scattered in the books, and if a DM doesn't understand how to use it right, a rogue turns into an invincible assassin. Stealth literally needs its own section of the book. I also really would love a good section in the DM guide for when "the rogue wants to scout all on their own" that helps allows the rogue to get some intel without spending the next hour of the game on the "rogue show".
6) Knowledges, especially Monster Knowledge: Again, the books are really vague on this, and its a major power difference. Some DMs will give you like 1 fact about a monster for a DC 30, and other DMs will tell you the creature's life story on a DC 20. That knowledge completely changes encounters, and there should be more guidance around it.
7) Monk: Can we finally get an edition where the Monk does not suck, or at least has the perception of not sucking? Now its a debate in 5e, but the fact that there are thread after thread after thread looking at DPR tables and Stun contributions and arguing whether the monk is good enough, is a problem in itself. Most classes are just cool enough on paper to be cool, they don't need stalwart "defense". But the monk continues to incite concern, so lets take a really hard look at this class and get it up to snuff.
8) Encounter Distance Table: Please give me back this beautiful little table from 3e. It gave you standard distances to start your fights at in different terrain (open field, thick forest, etc), and made accounts based on the party's perception (if one or both sides passed the DC, they encountered at distance, otherwise at half that distance....simple and easy). For people that focus on outdoor fights, this is a big deal and I would like to see that table reborn in 6e.