What to Bring to Ravnica

I have now read Guildmaster's Guide to Ravnica cover-to-cover. It does a great job of introducing the world of Ravnica to D&D, providing five new playable races, two new subclasses, ten guilds that serve as new character backgrounds as well as factions that can provide some significant rewards over the course of a campaign.

The book provides a lot of interesting ideas for long-term plots as well as really fantastic resources for one-shots (or episodic adventures that could fit into a larger campaign.)

But what can we bring to Ravnica that isn't there?

My first thought is races: Ravnica has merfolk and viashino - both Magic standbys. Their merfolk (who didn't appear in the original set - Vedalken were their main blue non-human humanoid race) are the variety that has legs and is amphibious rather than waterbound, so what I would do is simply include the Tritons from Volo's Guide to Monsters. I might alter the lore a bit - while the Tritons tend toward Lawful Good, Merfolk in Magic settings tend toward Neutrality (perhaps leaning toward Lawful.)

Additionally, Viashino are represented in some of the book's adventure hooks as lizardfolk, and I would again just emphasize a bit of the different flavor. While Lizardfolk are pretty emphatically true neutral as described in Volo's, the Viashino, being a Red race, are very much on the chaotic side of the alignment system.

There are monster lists broken down by guilds that mention stuff in this book as well as the Monster Manual, and also include a few Volo's and Mordenkainen's creatures as well, so you're actually pretty decently covered for that.

One thing you might consider doing if you're feeling really ambitious is to convert the recent Waterdeep: Dragon Heist into a Ravnica adventure. Swap out the Zhentarim for House Dimir, make the Cassalanters into Orzhov oligarchs, make the Breghan D'aerth a squadron of Golgari Ochran assassins, and perhaps make the Xanathar "Guild" a guildless gang with ambitions to be recognized as a true guild of Ravnica.

Another thing you might try is to expand into other Magic worlds. Using planar travel (or teleportation magic - I can't remember if Plane Shift allows travel between two separate worlds of the Material Plane) you might have your party leave Ravnica (maybe searching for Jace so that he'll come back and do his freaking job) and track him across other worlds like Innistrad, Zendikar, or Dominaria. There are Plane Shift articles for each of these worlds (and more) that are sort of precursors to the Guildmaster's Guide to Ravnica that could be very useful tools in this.

Jace does not have a stat block in the book, despite his important role in the world. I'd recommend that if you're going to have a planeswalker character, you'll definitely want to give them a high challenge rating (I think 20 at a minimum) and obviously give them the ability to cast plane shift innately at least once a day (if not at will.) Any planeswalker is probably going to be some variety of spellcaster, though not all have to be pure casters, like Gideon Jura is probably a paladin.

Plane-traveling villains from D&D might also find themselves in Ravnica. While there's no area equivalent to Chult as described in Tomb of Annihilation, I could definitely see Acererak building a horrible dungeon within the Undercity (though I think the most fervent Rakdos cultists would probably think it was just a really good fun-house.)

I also think you could re-skin a lot of Ravenloft adventures (like all of Curse of Strahd) as some kind of Dimir mind-prison - escaping a domain of dread might all be a kind of mental struggle against a Dimir Lobotomist trying to mess with your brain.

EDIT: Another thing I thought of! Slivers! In Magic, Slivers are a super-invasive symbiotic species that are kind of single-clawed serpents with horned heads that each provide their own enhancement to their fellow slivers. For example, a Winged Silver gives all slivers (including itself) the ability to fly, while an Armored Sliver might increase all slivers' health or armor by a certain amount (both values are amalgamated into "toughness" in Magic.) You could build a really interesting encounter in which the party needs to quickly slay the slivers providing the most dangerous abilities or perhaps have a fight against a very simple creature grow far more complex as more slivers with new abilities join the fray. This would require some careful planning and design work, but might wind up being really fun.

The book does what it needs to - it's not an adventure or particular story, but instead gives you a broad and diverse world that you will be free to fill with your own stories.

I hope that we see more of these sourcebooks come out (perhaps for more traditional D&D settings like Dark Sun, but updated for 5E.) But even if I'm very committed to my homebrew setting, I feel a very strong urge to run a short, couple-session adventure set in Ravnica. And if I can find someone willing to DM a full Ravnica campaign, I already have a strong concept for a Dimir Vedalken Enchantment Wizard ready to go.