A lot of what people are suggesting can be accomplished by just playing Dungeon Crawl Classics.
But in keeping with actually playing 5e, the big change I would like is ditching damage-dealing cantrips. Magicians being able to do magical stuff without using spell slots is fine with me - it addresses a major gripe my friends and I had about TSR-era wizards. But I don't like damage-dealing cantrips. It feels videogamey to me, and more than almost anything else makes D&D feel like a (barely) glorified combat game.