Well it's easy to focus on the negatives of WotC bring a terrible company. Never the less I think they did a few things very well that contributed to the success of 5E blowing up. Some of this is in concept even if the execution was off here and there.
agree with the most of it;
1. Advantage and disadvantage mechanic. Personally I was over the number bloat of 3E, 4E and Pathfinder. Hell throw in Star Wars Saga Edition. Mathematics isn't fun rolling dice is. Very newbie/DM friendly.
yes, but some granularity would also be nice, new exhaustion rules could be nice addition.
2. Level 1 and 2 being the training levels. Very noticeable running the game for newer players vs old salts. Sure we grumble about it but ENworlds not particularly representative or newbies.
100%, anyone else can start at 3rd or 6th level. or even higher.
3. Opt in complexity. Yes the champion might not be to exciting and we probably love our feats. A feats probably better than an ASI but ASI is still good in your prime stat early on. The champion might not be for you play something else.
IMHO, barbarian should have been simplest warrior class.
just two abilities Rage and move.
fighter should have been base class with battlemaster features.
same with sorcerer/wizard.
give sorcerer few spells and +1 per die for damage spells. You only have a hammer but it hits hard.
4. Bounded accuracy. Didn't work as intended but smaller numbers are good. Think I would personally prefer stretching +6 over 20 levels to +10 a'la 4E but use B/X stat array (18's capped at +3).
Disagree, I would even cap scores at 18(+4). Outside of magic items and high level spells, that is it. Raise secondary or take more feats.
5. The round structure. Very similar to 4E but you cant swap a move action into bonus (minor) action. This is good it's less complicated and you can dodge various exploits. You could use it to clone any previous edition tweaking it as required eg minor action becomes a bonus action and you can swap a move action.
I really miss giving up my move for extra Bonus action
6. Monster design. They kinda screwed up the monster design part but large using a d10 HD, huge d12, small d6 is quite good. One could also tweak the 5E monsters to redo older editions. Eg a B/X ogre being upgraded to 1d10 vs d8 HD is a buff that's not going to wreck the game. You could also use older edition design concepts eg 4E ones or adding old school energy drains and SR if you desired.
this should not matter as long as CR system works, but it does not.
7. Lower complexity in general. Got a laugh at my newbie friendly beginners game. I handed a 13 year old player 900 pages of the core rules and told him to make a character in 15 minutes. Instead I supplied each player with a 2 page cheat cheat summarizing the 5E round structure and what to do.
that you can make with every edition. maybe little more work with 4E
8. Excellent Starter Sets. Both the starter sets and essentials are very good for onboarding newer players. They all have their flaws. A great one woukd have LMoP (S toer adventure. B tier product)with a tweaked opening encounter, the bells and whistles of the essentials box and pre constructed characters from DoSI.
this should be every games goal.