One gripe I have with 5e the 4e I felt got right was the Warlock class using different attributes to cast spells depending on what they were casting.  I always felt that a 5e Warlock's Pact should determine their casting stat.  It might look something like this:
* Pact of the Tome - Intelligence
* Pact of the Blade - Choose Int or Charisma
* Pact of the Chain - Charisma

Something like that.

The martial arcane subclasses in the PHB (Arcane Trickster and Eldritch Knight) use Int as their casting stat, which is good and bad.  Good because they're "studying" magic, and that maintains the theme.  Bad because it suffers from multiple attribute deficiency.  I run an epic level campaign and our Arcane Trickster does rather well.  The 7th level ability Magical Ambush greatly offsets their MAD and lets them land power spells like Hold Person.