Q:

Alternate class system for MtG inspired setting

I'm trying to create some house rules to run a Pathfinder campaign in Ravnica, a M:tG plane which has 10 guilds, each one with a very specific philosophy and societal role.
I wish guilds to be :

flavorful mechanically-wise
optional (I guess I could make them mandatory, but it would be my last choice)
enforcing/rewarding guild's specific behaviour (roguish for dimir, violent for gruul, etc, but by rewarding, I don't mean offering reward, but "helping the PC as long as he acts this way") but...
allowing some room for different interpretations (not all dimirs are alike)
(more or less) balanced
offering increasingly powerful bonus/powers over time

Considered but rejected solutions :

full classes, because Pathfinder punishes multi-classing, and I think it would tend to violate my 4th bullet
feats, because they would be either not enough powerful to be flavorful, or too powerful to be optional

Considered solutions :

a bloodlines-like system (note : sorcerers are inexistants in Ravnica)
a 5-level class with prerequisites at every level, making them reachable at given levels if the PC took some feats and / or skills his guild favor

Is there any other rpg trying to do something similar ? Maybe Vampire : the Requiem ? I don't know it very much.

A:

What you're describing sounds a lot like the Factions from the Planescape campaign setting.
Here's a link to what has been done regarding Planescape factions for D&D3.5 : http://www.planewalker.com/pscs-chapter-3
The above document defines some PrC that are only open to members of a specific Faction,  and introduces a 3-level Faction Fanatic class.
As an alternative system, it presents the possibility of considering Factions as bloodlines from the Unearthed Arcana book.
The Faction-specific feats are defined in this document: http://www.planewalker.com/pscs-chapter-4

A:

I would actually vote for feats. Using feat chains or trees lets you retain the standard class system, saving you a lot of work, as well as balancing advancement in "Guild" benefits and abilities. Dragonmarked Houses from the Eberron campaign setting use feats in a very flavorful was and they are very similar to the Ravnica Guilds.
Some members of certain races are born with Dragonmarks, strange sigils that give them spell-like abilities. The Mark of Making allows creating or fixing material goods, the Mark of Healing grants, healing abilities, etc. Those with Dragonmarks have organized into Houses, which make up a huge part of the economy of Khorvaire, one of Eberron's continents.
Dragonmarks are taken by player characters as feats, which come in three levels of power. Each feat grants spell-like abilities that the character can use. (Such as mending, and so on.) You start by taking a Minor feat, then a Lesser feat and finally a Major feat. Each level of feat roughly equates to a spell level of the spell-like ability. (Minor is 0th or 1st level spells, etc.)
Modeling the Ravnica Guilds would work much in the same way. Each Guild would have a short (or long) feat tree. Minor feats could give skill bonuses or additional class skills, plus social effects. I'd probably create each feat to allow the choice of a couple skills to apply the bonus to. So the Selesnya feat might allow the choice to give a +2 bonus to Healing or Handle Animal. Each feat would also allow you to access Guild Resources, like safehouses, labs or Rakdos parties.
From there, the next feat in the chain could give a spell-like ability or an actual mechanical ability (something like Power Attack, Sneak Attack, etc.) If you really liked the idea, you could design combat-based feats and spell-based feats, allowing someone to play an Izzet fighter, if they really wanted to. 
Of your listed criteria, feats could meet:

1) The feats can contain as much guild flavor as you can come up with.
2) They would certainly be optional, and if you chose, you could give the appropriate 'minor' Guild feat for free to each player.
3) They would help PC's act in a manner consistent with their Guild, by giving them bonuses to doing guildly things.
4) If you offer options on each Guild feat (a list of skills to add bonuses to, a choice of additional class skills), then this one is met.
5) Balance is going to be a little harder, with the feats being home-brewed, but you could try to follow the spell levels that the Dragonmarked feats used.
6) Going back to the Eberron example, I believe that taking the second feat in the chain gives you the ability to use the first feat in the chain additional times. (For example, the Minor Mark of Creation lets you use mending 1/day. When you take the Lesser Mark of Creation, you get a new spell-like ability, plus you can now use mending 2/day.

A:

How about making each guild have a list of Favored Classes? Pathfinder has a fairly robust system of bonuses for taking your favored class, which are small but enough to count. Tell players that they may join a guild; if they do, they use the guild’s favored classes instead of their race’s. Or just remove them from the races altogether, so they only way to get them is to join a guild (and match the guild’s style).

