If so... meh?
Whilst it does have some merit, it also flies in the face of Magic's lore, where any kind of mystic can become a planeswalker, and also opposes the guidelines presented in Plane Shift: Amonkhet, where WotC directly stated they want to avoid the issue of "one PC is a Planeswalker and uses that to usurp all of the attention".https://youtu.be/GWfB7JzG1pw
[video=youtube_share;GWfB7JzG1pw]https://youtu.be/GWfB7JzG1pw[/video]
I like his idea of Planeswalker being a sorcerer origin, he makes a good argument for it.
Whilst it does have some merit, it also flies in the face of Magic's lore, where any kind of mystic can become a planeswalker, and also opposes the guidelines presented in Plane Shift: Amonkhet, where WotC directly stated they want to avoid the issue of "one PC is a Planeswalker and uses that to usurp all of the attention".
I think it's more likely that, if we get Planeswalking mechanics at all, they will be mostly story function, as per the PS:Amonkhet rules. A sorcerous origin derived from the planeswalker mechanic is theoretically possible, but Ravnica isn't Sigil; it's no planar nexus (that role goes to Dominaria, in MtG lore), so I would suspect such an origin is of low priority, if any.
I want to say that I doubt they'll reprint the Volo's Guide Goblin, simply because it doesn't mechanically mesh up with the Ravnican goblin to the best of my knowledge, and it's one of the races that were officially branded "Unbalanced" in Volo's Guide. Still, I am kind of cynical about WotC's competence in a lot of ways. But, we know from an earlier teaser that goblins will be a new race in the game.
Angels would be tricky; I'd like to see them added, but I wouldn't be surprised if they were instead used as a reskin for Aasimar, perhaps with some new racial feats. I can easily see Fallen (or is it Scourge?) Aasimar as the Black/White Angels of the Orzhov.
Merfolk? Possible on the one hand, but they're not a really "iconic" Blue Mana race in Ravnica. We might just be told to reskin Tritons for them. Still, we've had Merfolk in several Planeshifts, so reprinting them here in the GMG sounds safe to me.
Centaurs... I'd like to see, and I think they're one of the bigger races associated with the Green Mana guilds.
Nicol Bolas as a new Warlock Patron? Well, we actually had a really cool Sorcerer-King Pact for warlocks in 4th edition Dark Sun, so it's not unprecedented. This is something I'd never considered before, and I actually really like the idea.
Spell restrictions against planar manipulating spells...? Yeah, I could buy that, given the emphasis on how Planeswalking is special.
Dominaria WAS the Nexus, but the Nexus changed to a new location, I don't know where, perhaps Ravnica given that of all the MtG settings, it was chosen to be the one that becomes and official D&D setting in hardcover.
One thing to keep in mind, every setting is a product of the edition it was written for. In discussions about campaign settings, someone will always say "X doesn't belong in Y setting, because it was never there." Of course it was never there, because either it wasn't thought of yet or it wasn't part of the ruleset. Examples include: "Gnome paladins in Greyhawk are unheard of, sorcerers don't belong in Dark Sun, dwarves can't be wizards, dragonborn/tieflings/etc don't belong in Mystara." It's not that these things don't belong, they just didn't exist before or the rules didn't allow it to exist.
The way that WotC seems to be approaching things, if it's in the PHB, then it will most likely be in a campaign setting they revisit. They may say that something is rare or almost unheard of, but they won't say "You can't do the thing, because you're not allowed!" I've tried to take this approach when I worked on some conversions of Birthright and Dark Sun, because I know it's the approach WotC will take. And really, there's nothing wrong with that!
I sort of agree, but it depends a bit on the setting. Dark Sun and Birthright in particular went out of their way to mess with classes and especially races, in order to separate the setting from "vanilla" D&D. Most races are altered in those settings, both fluff- and crunchwise, and some are outright absent (e.g. gnomes in Dark Sun). Similarly, the discovery of arcane magic was a Big Deal in Dark Sun's history, and introducing a class based around a natural talent for it would feel wrong, though there you could at least refluff them as wasteland mutants of some sort.