There are certain types that are worthy of their own subclasses. There are "types" that D&D tried to make happen that simply don't work.
The point of divination and transmutation made in an earlier post are along my feelings. They aren't really something to build a single subclass around.
Essentially all magical practice in the world concerns these things...1) using magic to gain knowledge/"know" something, and 2) using magic to "change" things, idealized and mythologized as turning something -literally- into something; e.g., staves to snakes, men to pigs...or toads, take something heavy and make it light/float/fly, etc...
They are, basically, the function/purview of any magic-worker.
The "Illusionist/Enchanter," on the other hand, has a flavor of its own and magic that functions/does specific things to perceptions and minds.
The "Abjurer" is one of the first and most widespread type of magic user. The use of magics to thwart/protect from other magics, whether directed curses and the dark of night or the general ills of the world. Better name needed, certainly (D&D kinda mucked that up a bit from the get go), but an essential and - I would submit - necessary archetype in a magical world
The "Necromancer" is, of course, the one delving into those "forbidden arts," in large part/most settings, eschewed by most other magic-workers. Controlling the powers of life, death, and soul...beyond "the cardinal elements," beyond "trickery," beyond "what can be known...by 'normal/safe/accepted' magic."
A Conjurer, depending on one's flavor/definitions, could be just someone pulling rabbits out of hats or coins from behind ears. OR could be the demon summoner or call upon celestial tigers to defeat ones foes. As likely, one who "calls upon/up" (a.k.a. "evocation") a whirlwind or ball of fire. I have long thought Evocation/Conjuration to be of a kind. But D&D has striven, ever, to make the one about the "making/controlling of energies" and the other about the "creating something out of nothing or summoning creatures from beyond."
I don't necessarily think it that strong of an archetype to warrant its own subclass - by "school" of magic! BUT it is a theme for a subclass that could certainly work... not the least of which to provide an in-game "thwart/dynamic/dichotomy" between the "conjurers" and "abjurers" as I've done in my setting.
Similarly, Conjuration can be viewed as a natural "dark arts" kind of thing easily paired with Necromancers...just dealing in demons and devils instead of (or along with!) the dead.
So a "conjuring/conjurer" subclass seems appropriate just as a broader/more thematic subclass versus a "school-subclass."