A challenge: make a fort that could in theory last forever. Not just for a couple thousand years - forever.
The parameters involved make a significant difference. To start, I'm going to assume *completely* vanilla DF, with no mods, no hacks, no outside tools, etc. The other question is how much intervention from a player is required; my thought is that the player is allowed to set things up as needed, but once the "eternity lever" is pulled, there will be no further player input, and the fort is "scored" on a combination of how many centuries it lasts, and how interesting it is. Presumably the init files will be edited to remove all pauses, etc.
I'm going to skip for the moment any limits regarding memory, year rollover, integer overflow, or the like; those are in some senses external to the simulation.
I'm also temporarily assuming that while a fort consisting merely of an insane vampire in a cage (or some such arrangement) may technically continue to exist, it will score very little; the interesting question is how to do it with normal dwarves.
The primary long-term limitation is that nothing will be built or rebuilt, and no one will be assigned jobs; the only labors performed will be those that everyone does by default. AFAIK this means the dwarves will be eating plump helmets, and drinking well water. Additionally, dwarves will have to be graveyarded and left to rot away, which will be a significant morale hit. Assuming the fort is sealed off, it will need to be luxurious enough to keep everyone ecstatic despite that, because tantrums could be an eventual killer.. anything broken or overturned will never be replaced.
Once you've got your gold dining room for your dwarfs to eat their plump helmets and water in (and various other happiness measures), you need to worry about moods. My understanding is that moods are limited by a number based on the max number of dwarves, and the squares dug out. Obviously no more squares will be dug, and you can probably assume that part of setup would be to deal with a number of moods while still under player control. What I'm less certain of is whether with a roughly steady-state population fort, with dwarves dying of old age and others growing up, you will have a continual trickle of new moods. If so, this may *eventually* end your fort. An important question I'm not sure of is whether a berserk dwarf can destroy a field; if so and you get a slow trickle of moods, eventually that's going to be the unavoidable killer; no matter how many you start with, if they are never replaced, eventually there will be no more fields, then no more food, and then no more dwarfs.
Ghosts are another issue; they are poorly understood still. If dwarves dying of old age never leave ghosts, and moods cap out, then you're OK; there shouldn't be any accidental deaths to cause hostile ghosts. If old-age-ghosts can happen, but never possess living dwarves or damage physical objects, you're also OK, although the fort is going to get pretty creepy eventually ("the dead outnumber the living"). If moods don't cap out, eventually not only will you have berserking failed-mood dwarves, but subsequently angry ghosts; who may cause other problems and lead to a ghostly-aided tantrum spiral. Remember, under the circumstances, a table pushed over during a tantrum is a table lost forever. For "medium" term fortress, several hundred to a few thousand years, you could simply have "enough" spares of everything... golden dining rooms extending for entire Z levels, acres of barracks beds, huge arrays of compartmentalized 5x5 farms, and so on; in hopes that whatever damage the dwarves manage to do to their utopia isn't enough to bring it down. But if you are talking truly long term, anything consumed or destroyed will run out or fail some day.
My current assumption is that if you seal off before getting a baron, especially if you seal off before any caravans or liaisons come, that you don't have to worry about any appointed positions or nobility.
A recent problem will be clothing. You may need yet more opulent surroundings to cancel bad thoughts from eventual nakedness, as I'm not aware of any vanilla means to set up a perpetual clothing industry.