Because I suspect you specifically may have given the idea some thought...how would you look at the existing Subclasses through the M:tG Mana wheel, and what gaps do you think there might be to fill from that angle per Class?
I have given it some thought. I think many of the subclasses naturally fall into one of the 5 colours, some might be better suited to multicolour, and a few others I think could be fit a few of the 5 colours. I'll throw out some highlights of where I think specific subclasses would fit.
Barbarian I'd go with red (berserker, storm herald), green (totem), white (ancestral, zealot), black (zealot). Storm herald I think would at times be red/blue since anything dealing with the oceans or with cold fits blue not red.
Cleric domains fit well across all of the colours. Arcana at first thought sounds like it would fit blue but I think that most of its abilities fit white. If I had to pick a single colour, it would be white but if I could pick two then it would be white/blue. White has a lot of effects that end enchantments of exile creatures so it seems to fit with the main abilities of the subclass.
Most druids I would make green. A druid of the mountains could easily be red or more likely green/red based primarily in the spells it gains and might focus more on elemental spells over plant based spells. Circle of spores was made for the Golgari so that sits in green/black.
Eldritch knights focus on abjuration and evocation puts it in red/white but if you are allowed to mess with the schools, then I could see this subclass going into any colour except maybe green. Battlemasters focus on tactics and manoeuvres makes me think white more than anything but I'm sure it could be built around most colours.
While DnD dragons don't quite fit the MtG dragons, I would still place the draconic sorcerer squarely in red. It's the colour which has the most dragons with only a few existing outside it. Wild magic also seems to fit red with its unpredictability.
Warlocks seem pretty obvious where they would fit, demons are only in black so they get fiend pact, archfey fit in green, great old one though could be colourless if you somehow drew upon the power of the eldrazi. I think that would make for an interesting character. Celestial would be white and directly oppose the fiend pact warlock.
Wizard is mostly blue (divination, illusion, enchantment, transmutation) with some red (evoker, though focusing on cold spells and you fall into blue), black (necromancy), and white (abjuration). Conjuration, I think, could fit any of the colours. The subclass does have some teleport abilities, along with a lot of spells that do the same which is definitely one of blues things, but the focus on summoning in MtG would see me placing conjuration in any of the colours except green which would lean towards druid instead.
One subclass that I think would need to be made would be a wizard subclass for black that deals with enchantments. I believe they gave it the name dementia magic in the Dominaria set and it essentially deals with fear, and messing with the mind (through spells that cause you to discard, often hurting you when you do). The current enchantment subclass deals more with charms so fits blue more than black. Although you can just use a draconic sorcerer, I think that a dragon domain for clerics might fit well, or maybe it should be a druid circle, I've seen a few dragon shamans in various sets that I think this would fit well in a MtG game. I'd also like to see sorcerer subclasses focused on control of fire or cold. Chandra is a fire mage who I think might fit sorcerer more than wizard and there are a lot of blue spells that seem to deal with freezing and locking down creatures.
Other than that, MtG is so large that it's hard to think up any more specific subclasses. I wouldn't mind seeing another wizard subclass around being an artificer, more because Urza was one but he was also a powerful mage and the artificer being a half caster seems so limiting to how I imagine him. But then he's also an NPC so maybe that doesn't matter and he can just be built however the DM wants.
That was more written than expected.