If I had absolute dictatorial power, I'd
- Make all Classes pick their Subclass at Level 1 which would open up design space for Subclasses significantly,
- Rename the Monk "Mystic" and change Ki points to Spell Slots which they secretly are (among other changes to de-Orientailize them)
- Redesign the Ranger to have terrain be the Subclass choice (Forest Ranger, Desert Ranger, Urban Ranger, Underdark Ranger, Planar Ranger, Phlostigon Ranger, Arctic Ranger, etc.).
- Separate Barbarian cultural flavor from the Rage Class.
- Eliminate milticlassing and Feats. F 'em.
I don't expect such significant changes, however, when 6E does roll around. I do expect the Tasha's approach to Lineages to be used and expanded upon, which is good.