First off, props for the opening statement presentation, it’s probably one of the best I’ve seen here for a long time. Second, I’m going to join the cadre of rule breakers because I honestly couldn’t decide between the two which was more important to me.
1) Monster actions and diversity are sorely lacking. Even setting aside CR issues, there’re just far too many creatures in the basic MM that only have a default bite/claw attack to spam until their eventual death. In 4e, even the basic level 1 kobolds had a selection of 3 kinds of bullet for their sling, with different effects they could use. There were more zone effects, more save ends, and more auras/reactions. While 5e probably doesn’t need quite the same frequency, if I could be king for a day I’d try and make sure there was more battlefield diversity that didn’t stem from throwing caster levels on everything.
2) Significantly increase the number of non-magical class options (paladin/ranger/bard etc) and also give all non-magical classes abilities that were superhuman in nature, though still non-magical. Such as a barbarian stomping hard enough to cause fissures, rogues channeling their inner Neo to perform incredible dodges, fighters bisecting giants in a single blow etc. If D&D casters are going to end up stronger than a huge swath of fictional magicians, then martials should at least end up being on par with legends as well.