So, let's talk a little about subclasses. WotC tends to be particularly conservative about these, but, digging into Ravnican & Magic lore, there are a few possibilities that suggest themselves to me.
Azorius Senate:
Arcane Tradition: Hieromancy: This is a very prominent magical style used by both the Azorius Senate and the Boros Legion. Gideon Jura is a Planeswalker Hieromancer, albeit originating from Theros, who still has an association with the Boros Legion. It makes sense to bring this style of magic into D&D. To the best of my understanding, hieromancy is a White Mana-based magic form that relies on "enhancing order" and "suppressing enemy abilities". My understanding derives from the Magic wikia article on Wizards, and from the Magic article "Planeswalker's Guide to Gatecrash, part 3". Hieromancers are described the article as also creating spells that "sanctify or protect places or things". The Lawmage, whilst legally distinct in the Azorius Senate's hierarchy, seems to be a particular specialist form of hieromancer; they are described as "wizards who draw upon the binding power of the Guildpact to restrain criminals and then serve as lawyers once those criminals are taken to court", and who focus on creating spells that "compel or restrain beings". Taken all together - and there could be arguments for making Lawmage a seperate subclass, say an Arcane Paladin based on the Oath of the Crown? - I'd argue that Hieromancy could function in D&D terms as a "hybrid school", combining elements of Abjuration and Enchantment, similar to how the War Mage is based on an amalgamation of Abjuration and Evocation magic.
Elocuter: A literal diplomancer, an Elocuter is an Azorius specialist who focuses on communication, a diplomat backed by subtle enchantment spells. This could either be a new Bard subclass, or just the Ravnica name for a Bard in general.
Arrester: Azorius champions specialized in "stopping things before they happen", this could be a new Fighter or Paladin subclass, perhaps. I admit that this one is weak.
Boros Legion:
Wojek: I swear that I once saw the Boros Legion described as employing various magical means to counteract the abundance of magical creatures who walk the streets of Ravnica. I don't know if this is true or not, but a Fighter subclass - kind of like an Eldritch Knight that wields Abjuration and Enchantment spells instead of Abjuration/Evocation - could play into this archetype.
Pyromancers and Electromancers are both Red Mana wizards that appear on multiple planes, and do appear in the Boros Legion. These could be handled as seperate Sorcerous Origins - we got the "Embermage" (Kaladesh Pyromancers) as a Sorcerous Origin in Plane Shift Kaladesh, after all.
"Boros Guildmage": This would be another "magic hybrid" Arcane Tradition for wizards, representing the fusion of Red Mana and White Mana. Thus, it's a hybrid Evoker, focused on dishing out fire, light and radiant damage, and Enchanter, drawing from White Mana's association with mind-altering spells that compel harmony, and obedience... plus the Red/White hybrid spells associated with the Boros Legion, like Incite Hysteria.
Cult of Rakdos:
Riot Mage: This could be either a new Arcane Tradition, or in-universe terminology for a Rakdos-aligned Enchanter.
Bloodwitch: Again, possibly a new Wizard, Sorcerer or Warlock subclass, or just in-universe terminology.
Spiker: This is really just in-universe terminology for any Rakdos-aligned warrior, but, from the Cult's lore, I could easily see a Constitution focused Monk subclass representing the sadomasochistic, pain-fueled, lethally murder-entertainers of the Cult.
Madcap: This could be the aforementioned "Rakdosian Monk", or just in-universe terminology for Rakdos Bards.
Golgari Swarm:
There's a common supposition that the Circle of Rot Druid subclass from UA will appear here, and I agree, it makes a lot of sense. Other things I think we could see?
Rotbinder: An Arcane Tradition that combines elements of Necromancy and Conjuration. These are the fungus-shapers and bug-tamers, wizards who command both the undead and the lowly, creeping life that spawns most directly from death.
Lichedreg: A Fighter subclass for the Golgari, these are warriors who employ minor necromantic hexes to make themselves hardier, dulling pain and cultivating symbiotic fungi or vermin that repair their damaged bodies and attack their foes, a suitably dark and creepy but tanky warrior.
Gruul Clans:
I honestly can't think of anything new for these guys, except maybe new Totems for the Totem Warrior Barbarian. Maybe it's because of how straight-forward the Gruul are. Maybe it's personal interest, I don't know.
House Dmir:
We have a plethora of rogues and stealthy mystics in D&D already, so it's hard to see this Guild requiring anything new. I guess maybe a parkour-focused Rogue archetype, but that feels kind of weak. The Arcane Trickster is already capable of messing around with minds, so it's perfect a Dmir operative already.
Izzet League:
Chemisters and Blastseekers could be new arcane traditions... or they could be in-universe names for Artificers and Evokers, respectively. The Izzet League messes around with space, to my knowledge, but it's hard to see how that might function as a new Arcane Tradition. The Chronarch/Chronologist, a time-bending wizard, is well-represented amongst the Izzet League, but I admit I struggle to guess how that could be turned to a D&D class.
Orzhov Syndicate:
Most of what suggests itself for this domain is Cleric Domains, namely Avarice and Undeath, which reflects their obsession with drawing power from wealth and their mastery over thrulls and ghosts, respectively. Perhaps the Orzhov Knights could be the source of new Paladin subclasses.
Selesnya Conclave:
I don't really know enough about this particular Guild to comment, although their strange blending of nature magic and enchantment spells could perhaps be the root of a new Arcane Tradition for their Guildmages, or maybe a new Druidic subclass? Perhaps a Cleric Domain?
Simic Combine:
I'd love to see the Biomancer Arcane Tradition make an appearance, as this fleshcrafting wizard is the iconic Simic Combine member. Arguably, there could be potential in a Fighter subclass who grows increasingly spliced-up or grafted as they level up.
