As for missed opportunities, a few more things come to my mind.
Slashing/Piercing/Bludgeoning damage. It looks like 5e devs wanted to go somewhere with that, or wanted to keep a door opened for a further "module" of the ruleset, but never got very far with the concept of B/P/S damage. There was an opportunity to remove this concept from the game entirely - or to have B/P/S interact more with AC - but the final result feels like the leftovers of a good idea that didn't take off, or the incomplete implementation of a good concept.
Another missed opportunity was to give a more prominent and distinct role to the Investigation skill to 1) dial back the disproportional importance of the Perception skill and 2) give poor Intelligence a good reason to exist for all but wizard characters (ok, bit of an exaggeration here, but still... poor intelligence). Passive investigation, as suggested by the Observant feat, sounds like a cool concept, but one I've never seen being used in any game, including those I DM.
If class symmetry is/was a goal, there was a missed opportunity to have warlocks use INT as their caster ability. Then we'd have two INT casters (wizards and warlocks), two WIS casters (clerics and druids) and two CHA casters (bards and sorcerers). It would have fit so well! I know this is more aesthetic than anything really, but I like symmetry.
Going further with class symmetry, there are six non(full)caster classes, each with a (somewhat) defining key ability: fighter (STR), rogue (DEX), barbarian (CON), monk (WIS), paladin (CHA). Again Intelligence is left out, which *could* have matched a certain design of the ranger. Again, probably very low on the design goals list, but they *did* have the opportunity to make it so. Makes me sad. I like symmetry.