So it looks like someone made a similar suggestion about this:
http://www.bay12forums.com/smf/index.php?topic=56177.msg1214025#msg1214025The idea is, a forgotten beast (or mega-beast) arrives, and your dwarves don't think they can take it out. Or maybe they're just impressed. Anyway, you have an option to appease the beast by building it a shrine and assign it to the beast (if there is more than one beast on the map, you choose one for your cult).
Once you make a beast shrine, the beast behaves like a noble, making mandates and demands. If your fortress decides to worship the beast, then your relationship with the dwarf and human civs will erode (unless maybe they are evil beast worshippers too), possibly resulting in sieges to purge the land of your evil. Perhaps the siegers could give you a chance to renounce the beast worship before attacking. Maybe other civs such as evil humans, goblins or Kobolds could begin to worship the beast, and make pilgrimages to your fortress.
If you voluntarily destroy the beast shrine, the beast goes crazy and (tries to) kills the fortress. If something else destroys the shrine, the beast will give you time to re-build it before it goes crazy and kills the fortress.
The dwarves have to keep the beast happy; there could be many ways to accomplish this: build it a nice temple, give it nice things, show it adultation, make engravings or statues that flatter it somehow. Perhaps just seeing things die horribly could make it happy (you could be creative with this).
The beast makes demands from your dwarfs that enrich or feed itself. The 'shrine', which could either be a simple structure, or perhaps even better a customizable zoned room, would allow you to assign wealth and other objects to it, and these things belong to the beast's personal hoard. Also the beast might ask for animals or people to be fed to it, which you could also specify from the shrine menu. Possibly the beast could demand that visitors (merchants, etc.) from other places be fed to it.
Perhaps a 'grand priest' nobility title would be added, and the dwarf with this title would be the one who caries out services for the beast (brings it treasure, performs sacrifices, etc.)
Serving the beast, or even being around him often might cause dwarves to go somewhat crazy (perhaps the priest could be buffered against a full-on freak out, but he'd also go crazy in a non-debilitating way). However, the population would be required to worship the beast in person at least every now and then. In the shrine menu, you could specify a number of on-call worshipers, and dwarves would rotate in and out of worshiping duty. The beast could demand more adulation as part of its steady stream of demands, so the more arrogant or greedy it becomes, the more people it draws away from productive measures.
Dwarves can free themselves from the beast by killing it. Perhaps to prevent this from being too easy, the beast could become more powerful the longer it is cared for, or it could demand that dwarves not make certain items that would make them a threat (ie, demand that they do not produce steel armor, weapons, etc.) Building weird mechanical constructs near it's shrine/lair might also make the beast nervous. Also to prevent players from getting off too lightly, dwarves that have been devoted to the worship of the beast for too long might go insane or berserk if it dies.
Anyway, the older topic touched on this stuff, I just had the idea independently and decided to elaborate a bit. Hope this isn't all old hat...
BTW I'm not saying that this should be required by any means, but rather exist as an option to the already existing ways of handling beasts (killing them or walling them off somehow).