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. First up, one of the most fraught color combinations in design terms: red-green.
Some people are of the opinion that it's impossible to make red-green cards and characters interesting. The argument goes that blue's enemies together can't be smart. Emotion plus instinct apparently equals knuckle-dragging troglodytes who only want to eat and smash, all the time, every time. No planning, no thought, no concrete motivation. For better or worse, that's sort of what the Gruul Clans are. Borborygmos's guild does have a broadly defined mission for the good of Ravnica and some sense of purpose, even if it's muddled, because the loose confederation of tribes that makes up the Gruul knows that the preservation of nature is for a greater good. It comes across, however, that they're portrayed basically as barbaric comic relief or aimless, vagrant destroyers with no real function in the city except to make other's lives difficult.
So if we dispensed with the Gruul and made a new red-green guild true to the colors' core values - a smarter, more cunning one, still very free and mistrustful of authority but with green's focus on exploration and red's base greed, we'd come up with something like the Brangok Corsairs. Design credit goes to void_nothing on the MTGSalvation forums - aka myself.
The Brangok Corsairs are universally called "plunderers" and "opportunists" by their enemies. Some disdain the terms, preferring to think of themselves as explorers, while others embrace them - what's wrong with seizing an opportunity that the bounty of the natural world has placed before you? They are the guild of seafarers, archaeologists, and, yes, quite often pirates. Their leadership structure is quite democratic, with an elected Grand Admiral.
My travels. My treasure. My guild.
Landgrab (You may cast this spell for its landgrab cost. If you do, target opponent gains control of target land you control.)
Their keyword is about impulsive generosity. Red and green make for a convivial combination, and they both see the value in friends to the greatest degree among the colors, so naturally the new guild's keyword would want to play well in multicolor.
Landgrab 2R (You may cast this spell for its landgrab cost. If you do, target opponent gains control of target land you control.)
Plunderbolt deals 4 damage to target creature or player.
Brangok Toady GG
Creature - Ogre Warrior (Uncommon)
Landgrab G (You may cast this spell for its landgrab cost. If you do, target opponent gains control of target land you control.)
Rigging Rascal 2R
Creature - Goblin Pirate (Common)
1G: Rigging Rascal gets +1/+1 and gains reach until end of turn.
Indomitable Corsair RG
Creature - Human Pirate (Common)
Islandwalk, mountainwalk, forestwalk
"Forward, lads, let's get our names on the map!"
Rage-Current Rider 2(R/G)(R/G)(R/G)
Creature - Human Pirate (Rare)
Whenever Rage-Current Rider deals combat damage to a player, each player gains control of all permanents he or she owns but doesn't control. Put a +1/+1 counter on Rage-Current Rider for each permanent a player gained control of in this way.
"Freedom for us and a drowning for you!"
The Gruul Clans' take on the philosophy of red and green is a legitimate one; but there are definitely other ways it can be interpreted, as exemplified by the hypothetical Brangok guild.
Which Ravnica guild would you like to see redone next? Let me know in the comments.