Subclasses design space is virtually unlimited...
But what I think it's sadly missing from the PHB is more clerical domains and rogue archetypes.
For domains, the sky's the limit... pretty much any single "concept" can be turned into a "God(dess) of <insert concept here", so I'd like for instance to have domains to better cover some concepts used by deities of FR, and the historical pantheons: Wisdom, Justice, Duty, Agriculture, Magic, Death(good), Sea/Ocean, Mountains, Crafts, Arts, Beauty, Deserts, Murder, Lies, Disease, Luck, Undeath........
For Rogue Archetypes, just take a look at older ones in the playtest. A glaring omission in the PHB is that of a treasure hunter. Of course, all D&D characters are in a sense treasure-hunters, but IMHO the Rogue always more than the others, and the most iconic Rogue ever is Indiana Jones: so when I think of "Rogue" I always start from Indy foremost, and the problem is that he is neither a thief, nor an assassin, nor an arcane trickster.
Then another Rogue Archetype needed is that of a spy: the spy background doesn't help at all, in fact a true spy is someone who disguises herself as having another role in society, and that means it should have (or otherwise look like it has) any other background.
Possibly another missing archetype is "con-man" but in this case it might be that the Charlatan background itself covers it well enough.
In addition to those, I would like to see...
- more Totem Warrior animals
- a Bard subclass themed on being a dreadful performer (think Assurancetourix/Cacophonix)
- a tamer-or-beasts Druid (with plenty of animals, not the usual single "pet")
- as many additional Fighter Maneuvers as they can design
- more elemental Monk's abilities (not sure how many there are already, but I've heard they are not that many...)
Adding Sorcerer, Warlock, and Wizard new subclasses would be much easier than the other classes, so I'm not going to repeat other people's suggestions.
That leaves out the Ranger... it has only 2 subclasses, and honestly I can't at the moment think about more stuff beyond those (except that 3 favoured enemy types are really few, but that's not technically part of the subclass IIUC), except maybe a "planewalker" ranger. Terrain-based rangers could be another option, although IMO they could also do these with replacing favoured enemy with favoured terrain.