I was looking over the Development page and I was intrigued by the idea of Sects/Guilds, mostly Guilds, and I wanted to add my thoughts.
My understanding of the guild system is that if your fortress has a decent number of dwarves that have a certain skill level at a certain profession, they will form a guild and coordinate with each other. Let's say for example, that the profession in question is Mining, the certain skill level is Legendary 1, and the decent number is 3. So once your Fortress reaches a level where it has at least 3 Legendary 1 Miners, they will form a Mining Guild. Different professions like Planters might have different requirements, like needing more members with lower skill levels.
I mainly imagine the guilds in an adversarial role, something to give the player more to do and have more Fun. The player has to appease the guild members or bribe the guild leader in order to assure their cooperation.
Guilds could make demands on the player and go on strikes if their demands are not met. For example, the Mining Guild might demand a cabinet for all guild members, and refuse to do any mining jobs until their demands are met. The player might be able to bribe or threaten the guild leader into submission if they make demands that are unreasonable. How to do that would be tricky since there's not really a way to interact with an individual dwarf. One thought is that you might be presented with two demands, one that would appease the guild and one that would bribe the leader, for example, the Mining guild might demand a cabinet and chest for every member of the guild, or a tin cabinet and tin chest to bribe the leader (imagining in this case that tin is the leader's favorite metal) Or, it might be more Fun to arrange mysterious accidents or send in the Captain of the Guard to arrest dwarves that are striking. Ideally this would "scare" the dwarves into submission, with sufficiently brave dwarves unaffected. Besides striking, guilds might simply reduce output based on quotas, like carpenters make no more than 5 items a week, until their demands are met. Quotas might also be a means of appeasement, like if working constantly makes dwarves unhappy, making quotas could allow them to visit the tavern and temple.
Seems to me that the guilds should be able to elect their own leader, like the mayor, without player involvement. I would imagine that the election would be based on interpersonal skills, like the mayor, but guilds might also consider skill level at the profession or physical traits that improve the potential at the profession. So for example, the Mining Guild might elect a Legendary 5 miner with weak interpersonal skills over a Legendary 2 miner with stronger interpersonal skills.
Guilds might be able to demand a Guild Hall meeting area, or take over an existing meeting area for themselves if the player does not provide them with one. Guilds might demand quality improvements to their guild hall, like more tables, more space, higher value, or demand entertainers or drink servers. Non-guild dwarves that damage a guild hall, or even enter one, might earn the wrath of the guild.
Another thought is that the guild not only deals with the player, but with other dwarves. For example, if the Mining Guild goes on strike, it might not be up to the player to interact with the guild itself, but up to the mayor and guard captain to deal with them. If the Brewers Guild refuses to produce booze, a skilled mayor might be able to convince them to stop the strike, or a particularly intimidating guard captain might be able to threaten them back into action.
As suggested on the development page, dwarves that get into fights or arguments with a single guild member, could earn the wrath of the entire guild. There could be special anger triggers too, like the Weaver Guild will get angry with a tantrumming dwarf that knocks down a loom, or the Carpenter Guild might seek vengeance against a dwarf that knocks down a Masterwork bed produced by one of the guild members or even if your mining guild is on strike and you send a peasant to do mining work, the guild might get angry at the peasant. Anger need not lead to violence, a dwarf that vomits on a masterwork might simply be forced to clean it up, or an offending dwarf might simply be humiliated by the guild stealing their clothing, or the guild might steal personal possessions from the offending dwarf's room, or if a loom was knocked over by a dwarf in the Carpenter Guild, the weaver guild might knock over a carpentry workshop. I imagine that the nature of the punishment might be based on the guild leader or offended dwarf's personality, like a violent dwarf would be more prone to beating up offenders, a covetous dwarf might be more prone to steal, a generous dwarf might not punish anyone at all. It would also make sense for dwarves to develop a good reputation with the guild. For example, an offense against the spouse or child of the Mining Guild could also incur the wrath of the guild, or the guild could give them special permission to enter their guild hall or first pick of masterworks or whatever.
Since the artifact expansion will happen next, it would also be interesting to imagine some artifact interaction for guilds. A guild that produces an artifact might be able to lay claim to it, or they might demand that the artifact be placed in their Guild Hall or in one of their quarters.
The idea of guilds seems a lot like the traveling performance troupes, so it might be interesting for an entire Guild to try and immigrate to your fortress all at once.
There could also be rival guilds that cover the same profession. If someone gets kicked out of a guild for inappropriate behavior or arguing with the leader, they might be able to start a separate guild. Or if you already have a guild, a foreign guild could immigrate to your fortress and now you've got two of them. This could be advantageous to the player, if one guild goes on strike, the other guild might still be able to work.