Well, the subclass system doesn't really have a single purpose... on one hand, it's a framework to satisfy people with different expectations on a class, at least partially (e.g. not making wild shape a martial-oriented feature of all druids); on another hand, it's a way to cover character concepts, (e.g. Fighters focused on different styles) sometimes extending into something not generally covered by the class (e.g. Warlords); on yet another hand, it's a delivery system for optional mechanics; another is to create quasi-multiclassed concept; and yet another purpose is to dial on PC complexity...
With these in mind, really there are infinite possible subclasses to design.
Fighter: I'd be ok with ethnic subclasses such as Samurai, but they need to make sure they aren't picking features at random, and that instead the subclass feature are well representative of historical traits.
Barbarian: at least there is plenty of room for more animal totems, beyond that I don't know.
Paladin: I would stay away from designing paladins of each specific alignment, instead I would rather see different "good" ethos. There's already a lot of difference between a paladin whose main oath is to bring the guilty to justice, and another whose is to protect innocents or to free slaves and fight tyranny.
Ranger: technically the possible favored enemies are a lot, but the truth is that gamers have asked for a decade already to avoid monsters-oriented abilities because apparently they are afraid not to meet those monsters after all. Thus, instead of different monsters categories, we have to look at monsters types e.g.: big solo monsters, mob monsters, swarm monsters, flying monsters, swimming monsters, very fast monsters, monsters with lots of magical features...
Monks: some subclasses can be based on ideas from martial arts, but others can incorporate supernatural concepts and then there is a lot of room.
Clerics: domains are potentially infinite. I wish they made more difference however, domain spells should IMHO have more unique spells (or non-cleric spells at least) rather than just "always prepared".
Wizards: I don't really think all the 8 traditional schools need to be represented, but I do think that they should really feel different, not just a bunch of bonus spells. Summoner and necromancer (with small undead army) could be very interesting. Enchanter, abjurer and diviner could have non-spell abilities that are "always on".
Druids: I'd like to see an evil circle of druids which do e.g. some sacrificial rituals, a druid focused on having an army of animal companions, another with just a single powerful pet (but this may not need to be a subclass), and druids who gain permanent transformation into treants or other fey creatures. I don't like blighters tho... fallen Paladins make sense, fallen Druids not so much.