Big wall of text, sorry
Gruul: impulsive, chaotic, reckless, impatient, anti-civilisation.
Modus operandi: invade a part of the city, destroy everything and live off the loot until exhausted. Rinse and repeat.
Thing is: the modus implies that there are no lands for Gruul. Furthermore, translating the modus operandi to mtg would be something like target opponent sacrifices x (non-token and random) permanents (invasion and destruction), you sacrifice y (non-token and random) permanents (your losses in battle) and create z treasure and food tokens (the loot).
This does not sound like fun and is terribly difficult to balance. Maybe Gruul got shafted on interesting mechanics due to the flavour part being at odds with the core play style of mtg (lands).
There are only a few existing abilities that do fit: fight and the red temporary card draw. Madness (a great mechanic) seems like a fit due to the now or never character of the ability. However, it requires a lot of build around/thought to be able to use it and that is just not Gruul.
I think fight is a great mechanic, but it is build around creatures and I would love to have a Gruul mechanic which embodies Gruul, but eschews creatures. Is it possible to do something with the red temporary card draw and that is akin to madness? What if even better, the mechanic is generic but could be bended to the flavour of the guild?
The mechanic is:
entry: cost you may cast ~ for its entry cost from exile as though it was in your hand.
The cards:
Impulsive thoughts R
Instant uncommon
As an additional cost exile a card from your hand.
Exile the top 3 cards from the top of your library. You may play those cards until the end of your next turn. At the end of your next turn, put the cards exiled with ~ on the bottom of your library (including the one used to pay the additional cost and only cards that are still exiled).
Freed centaur 4G
Creature- mutant centaur Common
Entry 2G
When ~ enters the battlefield, destroy target enchantment or artifact. If you do, create a treasure token.
3/5
Raidfire 5RG
Sorcery mythic
Entry 2RG and exile a card from your hand.
Each player sacrifices 4 lands, 2 artifacts, and 2 enchantments. ~ does 4 damage to each permanent. You create 2 treasure tokens.
At the end of your next turn, put the card exiled with ~ on the bottom of your library.
The flavours:
The tempory draw (with returning the cards to the bottom of the library) fits the impulsive part without having to build around it like madness.
Gruul has cheaper entry costs than the casting cost, but often at the expense of resources like a card from your hand. This fits the use until gone.
Finally, the destruction for treasure.
For my second guild I want to use the same mechanic but then with a phyrexian twist. To show the inevitability of them. Yeah I know that they died for the trillionth time but hey, they always come back.
Orzhov: a religious cult that desires wealth (which they think means power) above anything else. They do this partly by being the loanshark. The other parts of having people indebted after dead, making them into thralls, the self-sacrifice of their followers to get to their goal fits quite well with phyrexia. Let's focus on loanshark and the endless phyrexian wave...
Orzhov loan officer 1W
Creature-human paper-pusher (argh, I mean human advisor) common
Entry: 3W and pay 1 life.
As ~ enters the battlefield choose a player.
When ~ enters the battlefield, the chosen player creates a treasure token.
At the beginning of the chosen players upkeep, that player pays 1 or 1 life.
1/3
Phyrexian nighthawk 1BB
Creature- phyrexian vampire uncommon
Entry: 2BB and pay 3 life.
Flying, deathtouch, lifelink.
When ~ dies, exile it instead.
2/2
Orzhov Arena 1WB
Enchantment rare
When ~ enters the battlefield every player creates 2 treasure tokens.
At the beginning of every players upkeep, that player pays 1 or loses 1 life.
At the beginning of your upkeep exile the top two cards of your library, exile a card from a graveyard, and you lose 1 life.
The flavours: loans which have to be paid back. These loans slow the game down or whittle the life totals, which is in favor of the phyrexians since they play the long game. The entry costs are often more expensive and require life, but some, especially the phyrexian exile themselves when they die.