Stats: There are too many of them. It needs slimming down to maybe four. These same stats, and what they denote regarding lifting, running speed, healing rates are not good. They could take a cue from Shadow of the Demon Lord and Lords of Gossamer on this.
Magic: Everyone has it, and spell slots. Both need scrapping. Aetaltis uses power points, and allows players to cast above their level and risk damage etc.
Class Powers: Savage Worlds takes the lead here with the Mystic Powers category. In older Runequest these were called Legendary Abilities, which became Heroic Abilities.
Not all classes need powers, nor should they.
Classes: There only needs to be three core classes. Or even Nobel at all to allow each setting to be completely bespoke, instead of feeling compelled to insert nebulous power subsets, Barbarians with various paths.
Levels: Not required, just have tiers of play.
Magic-Users: We dont need three different versions. Each could be covered with an Edge / Feat.
Powers: It badly needs a low level supers game that works, as ALL of them shoehorn spells as powers which is utterly terrible. As such it would be better to run M&M 2e in order to covert with the least amount of fuss. Modern Age does this in their Companion.
Skills: Would be better importing from Doctor Whomrpg, 1e or 2e. Then have specialisations or Knowledges, which 5e already does.
I don't recall 5e listing what the +bonus in skill levels denote, whether Olympic, world class etc. M&M does though.
AC: This only really lends itself fantasy games, as soon as science fiction is added you get problems. No standard DnD character would survive laser weaponry. Plus science settings require high levels of damage, something obviously done with Savage Worlds Rifts and Savage Pathfinder. Also covered in a 3PP in Dark Matter with mega-damage, mega-creatures and mega-magic.
Cosmology: It is terrible. With some thought they could get it to work with any setting yet they constantly stick with the retarded current cosmology.
Before anyone says I must hate 5e, I just acknowledge the problems and fix them.
Also needs a narrative state of play too, like Lords of Gissamer.