Core D&D needs more mundane class options than fighter, barbarian, rogue. Enough of this "spells for everyone, whee!" design philosophy! The knight and scout both seem popular, the ranger really ought to be mundane, and why don't we have a Noble class in the core rules yet?
D&D needs a wound threshold and a formal condition track instead of a massive damage threshold and a laundry list of conditions that have little to do with each other (dazed, confused, fatigued, exhausted, nauseated, drowsy, cranky...). I think that would both
simplify combat and make it more cinematic, as well as adding another way of imposing
a cost on magic.
D&D needs a unified theory of spellcasting. Have one chart for prepared casters, another
for spontaneous casters. Give each class a progression on that chart (e.g. +3/4 caster
levels per level for bard-types), along with associated spells per day and spells known.
Allow even mundane levels to improve spellcasting, provided the character already has
the magic. (So the fighter 10 who gains a level in Wiz doesn't suddenly cast as a 3rd or 4th
level wizard.)
I think those three changes would greatly improve the game.