Short version:
>Fighters
All archetypes get combat maneuvers. Maybe not allow the same manuevers to be used twice in a row just to cut down on precision attack and the "power attack" feat spam.
>Bards
Remove their flavor focus being on music, leave that up to the player. Hedge mages, Witches, wandering Sages, and such are cool flavor. Make em support wizards and take away some of the regular wizard's way-too-many options.
>Barbarians
Rage no longer gives exhaustion levels.
>Druids
Remove wildshape. Yes, I know the point of the class is wildshape, but mechanically it's kinda shitty to have a character who spends all their time playing stat blocks of other creatures instead of their own character-build. The scaling is also broken as hell.
>Monks
Un-fuck the Way of 4 Elements Monk by making spell costs/action economy more reasonable. Maybe give Way of the Shadow more skill proficiencies to better fit their "assassin" or "ninja" profile.
>Rogues
Give rogues some kind of resource pool that can force Advantage or pull off tricky manuevers, so they're not screwed the moment they lose a flank-buddy.
>Rangers
UA already did a good job of this.
>Wizards
Bring back opposition-schools tied to the school specializations. Make Evocation a bit more powerful, move some of the "save or suck" and control spells into the Bard and Druid spell lists exclusively. Remove all spells that flatout replace entire skills or do them better than classes specializing in those things (Alter Self for Deception, Invisibility for Stealth, Create Food and Water for Survival, Knock for Lockpicking, ect).
>Warlocks
Make short rests 5 minutes
Hex is now an Invocation that the warlock gets for free at some point.
Repelling Blast doesn't work on creatures of Large or larger size (sorry, but knocking around a full-sized dragon with your finger-guns is retarded as hell).
Stop trying to make Warlocks into fucking gishes, Paladins and EKs already exist for this, goddamn.