Voted for Psion. Even though I loved the Warlord, there's just too much healing already in 5e.
With the Bard, Cleric, Druid, Paladin and Ranger all able to do at least some healing, and HD, and healers kits & whatnot, there is rather a lot of healing in the game, already.
OTOH, with Bard, Cleric, Druid, Paladin, Ranger, Wizard, Warlock, Sorcerer, Monk, EK, AT, & Totem Barbarian
all wielding supernatural powers (mostly as spells), there's a lot more supernatural powers, just in the PH, already, too.
A third* class with multiple non-casting sub-classes would not be amiss.
An additional class with supernatural power not strictly spellcasting also wouldn't be so bad, either - though there are actually a lot of such powers spread out over a lot of classes, they almost all come with actual spellcasting, as well.
sure, lots of those have previous things that could be considered sub classes, but I dont think those really fit anything close to the 5E concept of class and subclass.
No? Because the 5e concept of class & subclass seems very flexible, if we're to go by the PH classes.
The Rogue & Fighter have de-facto multi-class magic-users as sub-classes, for instance.
The Wizard's 'traditions' are just the old schools, which merely group magic spells roughly by what they do, and, while former ed specialists were actually focused on their school to the exclusion of at least one other, every 5e wizard can use any spell of any school, they're barely differentiated from eachother, and they're all the exact same bookish concept.
The Clerics' Domains are also pretty utilitarian, that way, just what is your god a god /of/, you're still just a worshipper thereof. No big differences.
A Psion could, without even getting into the range of concepts in past editions, just divvy up psionics functionally into disciplines and sciences by similar criteria and have 9 or more of sub-classes at
least as valid as the Cleric's & Wizard's.
Remember, 5E takes a broader approach to things and if fluff and flavor can handle it, then no need to make it unique in 5E.
That can't be true, or we wouldn't have separate Druid, Sorcerer, Bard & Warlock (and Paladin & Ranger, and Barbarian and Monk) classes. The Big 4 could handle everything that way.
A big part of the success of 5E is its lack of proliferation of options
I don't think it's any one thing - or anything, at all, to be found between the covers of the PH - that can be tagged as a major part of D&D's revival, at this point. IMHO, it's a cultural phenom, just like last time, and most of the factors - the big parts of that success - have virtually nothing to do with the game. 5e could be a re-print of the 2e core books or the Red Box & the Rules Cyclopaedia, and it'd be doing just as well.
What we want can be handled by homebrew and distribution on places like here and the Guild. A glut of options may make us happy, but in the long run I think it's pretty safe to say such would hurt D&D.
What class is next isn't a glut of options - the plethora of extreme tenuous-concept, niche sub-classes and oddball races we've been seeing, maybe. But psionics is not just more fiddly choices sub-dividing the same things, it's something outright missing from the game. Same for the Warlord. Arguably the same for the Shaman and even some of the others.
* after fighter and rogue - a class with
no spellcasting, even a sub-class, is clearly too much to ask of 5e.