[5E] Non-Wizard Arcaners in Glantri
by Giampaolo AgostaBards
I think this is a point where 5e departs heavily from the interpretation of magic of BECMI and AD&D.
In those editions, arcane magic is basically unable to perform healing. This forms the basis of an important political plotline of Glantri, which I would be reluctant to give up -- indeed, I've kept my 5e Mystara campaigns outside that nation also to prevent this particular clash.
I'd probably take a cue from Elven magic (which includes typically arcane spells, but is actually enhanced by Ilsundal to allow Elves to case Cure Light Wounds and Cure Disease, albeit at higher levels), and rule that Bards are actually semi-divine (or, maybe, semi-druidic) casters in Mystara, probably powered by M-Celtic Immortals like Diulanna.
That would address the issue fully. Like Elves, these characters would be accepted as wizards in Glantri until their actual nature of Immortal-worshippers is discovered. They would also not be able to learn spells on their list that are not also present in the Wizard list, except via personal research or travelling abroad.
Sorcerers
Sorcerers also have some peculiar interactions with Mystara's lore. In principle, BECMI Magic Users represent both Wizards and Sorcerers (at least), with some Outer Planes being rich in magic and presenting NPCs that have Magic User stats that model innate spellcasting (i.e., they would be Sorcerers in later editions). In particular, Old Alphatians lived in a magic-rich environment, where Sorcerer powers were likely the norm. Possibly a less talented (or just scholarly) individual would operate as a multiclass Sorcerer/Wizard, down to pure Wizards for people unable to perform innate casting.
Most Alphatian "wizards" would likely cultivate any innate spellcasting capabilities they have, but, given that their overall spellcasting ability decreased significantly in the relatively magic-poorer Mystara, Sorcerers (particularly single-class ones) are likely uncommon in Mystara (but certainly treasured -- Alphatian noble families would likely try to strengthen their own bloodline by having offsprings with such individuals).
Note that the Divine Soul provides healing magic to Sorcerers, but those are still innate abilities, so they would be accepted in Glantri. Divine Soul Sorcerers would be the 5e translation of Progeny, if you use those.
In Glantri, the Sorcerer would be the typical talented but lazy student who steamrolls through the exams without actually studying much. Most would at least multiclass to Wizard to a small extent, particularly if they graduate from the GSoM, representing their more formal training (luckily spellcasters multiclass better in 5e than in 3e or 4e, so this should not be a terrible issue).
Warlocks
Warlocks cast mostly arcane magics, but their source is external. They would be treated as Clerics in Glantri, or even worse, since their patrons are mostly powerful outsiders. Celestial Warlocks are not that different from Clerics, anyway.
Outside Glantri, Warlocks might be more common, since there are many powerful entities that mirror the 5e patrons, including the Fiends, the Elemental rulers and the Genie rulers, and obviously the Outer Beings.
Artificers
Artificers are somewhat like Bards, but easier to deal with, since they don't actually cast spells except in rules terms -- they merely produce spell-like effects through some other technique.
I'd guess they are very rare, but might be employed to model any number of exotic techniques, including the Hakomon sigil magic, the Secret Craft of Alchemy, and the demihumans' ability to create magic items without actually being magic users.
Luckily, Artificers multiclass easily with Wizards, so a Glantrian Alchemist would likely be a multiclass Wizard/Artificer.