I would tweak Legacy of the Void units. How the heck can they give us other Broodwar classic units like Dragoons and Reavers but not the Scout (even though the Taldarim still use them). Artanis in BW was a scout, if you are gonna go down the nostalgia train with dragoons and reavers, you got to include scouts.
Phoenix and Mirage too similar, just give Phoenix double lift and phasing armor make purifier unit, and give us the Auir Scout.
We could also use more unit selection in HotS, and it be nice if HotS selections weren’t permanent. Would be nice to train primal zerg variants, and have option of some classics BW units like Guardians.