The main change I'd like to see is stronger direct damage spells, but postponement or weakening of resistances, movement abilities like flying, teleport, etherealness, etc. Also, divination should be weakened so that there is always uncertainty and easier ways of concealing the truth. And I think absolute defenses or unstoppable attacks should be avoided. So for examples, spells without saves, or spells that are generally unstoppable by creatures of appropriate CR should be avoided. I don't like the way that relatively early on spells turn medieval adventurers into flying, teleporting superheroes resistant to most environmental hazzards. I prefer more of a sword and sorcery feel which keeps environment and physical skills like climbing, jumping, hiding, etc. relevant in the game.
Spells like true seeing, mind blank and hero's feast which make creatures immune to illusion, enchantment and poison, respectively, should be avoided or weakened so they merely give strong pluses to saves or similar effects.