No idea if this has been posted yet or exists in the cores/reqs/bloats, but here goes.
Eventually, once they are fleshed out, deities and religions may very well create factional groups, causing interspecies, or even intraspecies tensions. I certainly hope that they do--these factional groups could certainly add to gameplay, especially in fortress mode.
POWER GOAL-ISH OUTLINE: Dwarves join factional groups depending on occupation, religion, class, and other factors, resulting in potentially complex interrelationships and interactions between factions within a single geological area.
For example, all of the guilds (to be explained) in a fortress form a bloc with the followers of the deity of "labor and sacrifice" and the poor dwarves, and initiate hostilities against the dwarven nobility, administrators, and guards. These hostilities result in meaningful, and yet hilarious violent interactions between groups, potentially overturning the dwarven economy and reverting to dwarven communism (pre-economy). Or maybe the guards just slaughter all of the workers and priests, dooming the fortress to a slow death from lack of resources (because additional migrants are frightened away by the dwarven mass-murder).
Also, factions might seek retribution for an assault against a high-ranking member of the faction.
Sounds great, right?
Well, in order to get these factional interactions happening, the game needs a factional system. In this factional system, each dwarf has an "allegiance meter" for each one of the factions, determining which other dwarves he will side with in the event of a conflict.
FACTIONS in a fortress could be viewed through a faction menu, displaying the names of each faction and its members. Some factions might be unnamed and not list their members if they feel that secrecy is in their interests. Otherwise, one of a faction leader's jobs would be compiling a roster of the members of that faction, something like the current job of bookkeeping.
In order for this factional system to be more than just "poor vs. rich" or "god labor vs. god of parties," fortress mode needs some more interesting institutional frameworks. So, religions are a good start, as are nobility/administration/loyalists. Another idea is GUILDS!
I say, bring back the guildmaster nobles from the 2D version, but make them have a purpose before adding them--their purpose should be leading actual GUILDS! So once a guildmaster arrive, perhaps he demands a building be made to house his guild--maybe designated from a workshop of his trade, rather than from a chair or table. The way the dwarves meet with the mayor in the current version, they should meet with the guildmaster of the guild they are in--then the guildmaster can meet with the mayor on behalf of his guild. Dwarves would gain status in a guild once reaching "unmarked status" in a particular trade. Dwarves could be members of multiple guilds, although they would ultimately side with the faction with which they have the most allegiance.
Another institutional framework: HOUSES! Dwarves descended from a particular lineage should be de-facto members of a particular HOUSE. A HOUSE would be formed once a fortress acquires a certain number of dwarves from a particular lineage. Entire migrations of dwarves might all be from the same house, relocating itself to your fortress in search of riches! Each house would have a patriarch or matriarch, leading the house. Some houses might function like a dwarven mafia, while other houses might revolve around more acceptable, political aspirations, like securing appointments within the nobility, administration, and mercantile classes.
The MILITARY itself could function like a faction, with dwarven champions deciding to lead a coup against the nobility/elected officials if they are displeased with their pay and deployment.
Any other ideas for institutional frameworks that would result in factions? Any critiques of the idea?