Last time I did a guild rebuild - and that was a long time ago - I chose to focus on white-blue and presented the Granen University. That was the fifth new guild I made, and I'm not about to quit exactly halfway, so here's the intro again:
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.
Injecting a little bit more blue into the system, I'll be doing an enemy color combination this time - blue-red.
Blue and red are notorious for not getting along, creature-wise, other than the fact that they both are "supposed" to get fairly bad small creatures (blue because it shouldn't be too fast, but rather deliberate, and red because red gets efficient burn to deal damage early on). There is no real creature keyword that blue and red share, other than a general taste for evasion mechanics in both, and occasionally "target creature with power N or less can't be blocked this turn" effects. Noncreature wise, they don't agree on very much else, except for looting, and both getting spell copying and redirecting (although one or the other has been exclusive to either blue or red at various times). Blue and red both have some notably disruptive archetypes - blue with spell countering, and red with land destruction - and the two have historically gone together in "counterburn" decks.
Like the other enemy-blue guild in Ravnica, the Izzet League are scientists. Mad scientists, even - the Izzet in this case represent the exuberant cackling technobabble Einsteins with little regard for safety procedures and a penchant for explosions. This is fun, but it's a schtick, and it's hard to relate to - after all, they're only "mad" because they're purposely written to think in convoluted ways no one else can understand. On both occasions in Ravnica blocks, the Izzet have been the "spell guild" - both replicate and overload are instant and sorcery mechanics, and were considered fine and good. There has been some demand for a more artifact-focused Izzet, but since I already have that covered with another guild in my rebuilds, I'll go with another angle altogether.
Rather than combining blue's inquisitiveness with red's reckless emotion, why not add blue's dispassionate self-collection to red's thrill-seeking and daring? Ravnica is a big city, after all - one of the biggest in all of fiction, by some measures - and accidents and deliberately set disasters are bound to happen. Who saves those who were in the wrong place at the wrong time? In the real world, it's the fire department. On Ravnica, it's the Hebyt Squad.
When danger's near - fire, flood, earthquake, blizzard, or magical catastrophe - you can bet the Hebyt Squad will be around to. This is in part due to their guild's charter, which charges them with the preservation and rescue of anyone caught in a natural or unnatural disaster, and in part because they're all natural adrenaline addicts. Hebyt members will use all manner of trickery and gadgetry, both magical and mundane, to save lives and property in the most daring way possible. They can be found teleporting out of the sky into infernos, or using explosives to divert a raging deluge.
The Hebyt have a complex system of ranks and honors that seems impenetrable to outsiders, and are usually somewhat self-aggrandizing and lonerish by temperament, but one thing is clear - they all defer to the dashing and caped Swift Monarch, Celdna, who never shows their masked face.
Guild mechanic - Recombust X (Once per turn, if a source you control would deal damage to a permanent or player, you may return a permanent with converted mana cost X or more you control to its owner's hand. If you do, it deals that much damage plus X instead.)
Hebyt Raid 1UR
For each player, you may return target creature that player controls to its owner's hand. Then Hebyt Raid deals damage equal to the number of creatures returned in this way to target player.
Viscous Fuel 3U
Sorcery spells you cast cost 2 less to cast.
Recombust 1 (Once per turn, if a source you control would deal damage to a permanent or player, you may return a permanent with converted mana cost 1 or more you control to its owner's hand. If you do, it deals that much damage plus 1 instead.)
Skyboot Elite 1URR
Creature - Human Wizard Soldier (Rare)
Whenever Skyboot Elite attacks, reveal a card at random from your hand and play with that card revealed until end of turn.
Whenever Skyboot Elite deals combat damage to a player, until end of turn, whenever you play a revealed card from your hand, return target permanent to its owner's hand and copy the next sorcery spell you cast this turn. You may choose new targets for the copy.