I've long been fond of the idea of simply improving the less-used mental skills:

  • Charisma
    • Intimidation: Anyone can attempt to cause the Frightened condition via Intimidation. This should have suitable nerfs to make it worse than doing the same thing with a spell - for example, its duration should be at most letting the target roll to resist every turn.
    • Performance: Spellcasters can make a Performance check to obfuscate the components of a spell as singing (for V) or a physical performance, like dancing, for S and M. Observers must win an opposed Arcana check to recognize the spell as a spell.
  • Intelligence
    • The various Lore skills (everything here except Investigation) can be used to recall information about observed monsters without the DM needing to invent a rules framework for this from whole cloth. Make sure to have Arcana do the least of these, as it already does the most in other contexts. Examples include damage resistances and special attacks.
  • Wisdom
    • Animal Handling gains general rules for communicating basic, simple concepts via body language and tone, without any requirements about the target having a given type or being your mount.
    • Medicine now lets you diagnose the specifics of anything resisted with a Con save, and you can use your Reaction to replace a Touch-range ally's Con save with your Medicine check.