5e's problems as far as I can tell are:>Tries to be streamlined AND granular, fails at both.
This is why I think people keep making the 'make it more like 4e' argument. 5e basically tries to have it both ways and it just... fumbles.
Its handling of equipment is actually a good example: given the choice between a flail or a longsword, why ever pick the flail? the longsword does the same damage and also gives the option of 2-handing for d10.
But you might say 'bludgeoning damage vs. slashing damage'. Problem with that is, the way the game's set up, you could just replace every instance of 'bludgeoning, piercing or slashing damage' in the game with 'physical' and it would effect maybe 3 monsters (and that's being generous) and rules regarding breaking out of nets - a weapon so weirdly particular and restrictive in the RAW that it's a waste of time for any character level 5 or above to bother with them.>Skewed ideas on character balance
Race balance is a mess.
For instance: It seems natural weapons that deal 1d4 damage are held to the same character power as 1 point in an ability score. Okay, that sounds reasonable- oh wait, nevermind, improvised weapons are generally assumed to be 1d6 clubs. >Inconsistent class quality
What I mean is that not all classes are insensitivised properly. The game says you're supposed to get Wis as a Ranger (when Dex and Con are better investments).
Also, options during bonus action are treated as a big opportunity cost, but not all classes even have features that use them.>An insistence on making new shit all the time rather than patching the old.
Xanathar's section on making use of tool proficiency was fantastic and addressed a gripe I had with the game... and then in Mordekainen's they lovingly inked the section on Eladrin with their own jizz.
So yeah, if I were making 6e, my focus would me on addressing these points. I'd make the game more granular though, every fucker under the sun has a streamlined system these days.