I re-introduced my last guild rebuild, the red and white Toval Union, with the same thing I did for my first:
Ravnica is an immensely well-regarded plane, and most of its fans love it because of the guild system, allowing each of the ten two-color combinations to get relatively equal representation, its own identity, and a new keyword. And the ten guilds do capture the essence of what their colors are about, philosophically and mechanically - or at least part of it. Individual colors are deep enough that their philosophical essences can't be pinned down in one word, of course, and so are the intersections of two colors. And it becomes problematic when players think that all two-color cards, concepts, or factions need to fall in line with the appropriate Ravnica guild's beliefs.
So, as a thought experiment, and partly as speculation for the future of multicolor sets and perhaps even Ravnica itself, I will propose versions of Ravnica guilds that might have existed if R&D had chosen to focus on different aspects of the colors that make them up.
Given how fun and enlightening this has been, let's continue with another enemy color combination - black and green.
Mechanically, black and green are about as different as two colors can be. Green is much better at creatures than black is; but only black is good at bringing them back from the dead. Green can't destroy creatures nearly as well as black can; but it compensates for that by being able to permanently remove any kind of noncreature permanent. As a matter of fact, the one major overlap these two colors have is caring about the graveyard, and the canonical black-green guild, the Golgari Swarm, capitalizes on this by focusing on the dichotomy of life and death. Their flavor always tended to lean more black than it did green - so much about the Golgari is unnatural in the conventional sense of what "nature" is, and as cool as Plant Zombies are, just because they have phyto-components doesn't make them much more green than undead are (which is not much in the first place). The other problem with the Golgari has usually been mechanical: Dredge is considered one of the most inherently broken keywords of all time, and scavenge, while serviceable, is frankly kind of boring.
So what angles could black and green tackle other than the necromancer cliches? Well, something both black and green often are that gets ignored is predatory - black out of opportunism, green out of instinct. What if the black and green guild were the hunters of a civilized society? That was the premise behind my own design of the Tulgri Hosts.
The Tulgri Hosts are the finest trackers and hunters anyone could hope to hire. Untiring and with uncanny senses, a Tulgri ranger can track their fleeing quarry to one end of the plane and back - and whether the objective of their hunt is a rare bird, a rhinoceros, a priceless diamond golem, or a human being is of no consequence to them. They'll bring back anything, dead, alive, or otherwise, if the price is right. Their leader holds the title of Keengrim, and the Greatstriding dynasty of nantuko has provided various prominent Keengrim throughout the ages.
My pursuit. My prey. My guild.
Keyword action - track (To track, choose an opponent and a number. At the beginning of that player's next end step, if that player controls that many nonland permanents, the track is successful.)
Unsettling Footsteps 1B
Target player discards a card, then track. When this track is successful, that player discards an additional card. (To track, choose an opponent and a number. At the beginning of that player's next end step, if that player controls that many nonland permanents, the track is successful.)
If it's hard to shake the sensation you're being followed, it's because with the Tulgri you can never tell when you are.
Urrtmuzzle Grappler 3B
Creature - Human Warrior (Uncommon)
At the beginning of your upkeep, track. When this track is successful, put a 2/2 green Bear creature token onto the battlefield under target player's control.
5G: Put a 2/2 green Bear creature token onto the battlefield under target player's control.
"The assassins have it easy - all they gotta do is find and kill. Finding a bear's never trouble, but then you gotta capture it."
Keengrim's Bowmaster 2G
Creature - Insect Archer (Rare)
Creatures you control have "Tap: This creature deals 1 damage to target creature with flying" and "2B, Tap: Destroy target creature that was dealt damage this turn."