Disclaimer: I'm not going to read through 100 pages of posts to see if these things have already been said. These are my thoughts, and some ideas that I am sure other people have already suggested. If you're interested, enjoy. If not, move along. :-D
Is there really any point in having a scripted endgame at all? Currently, there is no win-condition, and its relatively easy to avoid hitting the current end-game trigger (by simply murdering any king who suggests otherwise). Adding a new end-game trigger and sequence seems non-trivial, and its mostly content that we all avoid until we're either bored or finished with a game. Its not a surprise to anyone who reads the wiki (which you pretty much have to do to play the game) and I have not heard too many people complaining that the endgame was just too tempting.
I've never reached it myself, but I have heard a lot of people say they've avoided it altogether, or that you can defeat it (as mentioned previously in this thread). By doing that, sure, you might eventually run out of things to do, but that's sort of your prerogative - just like building the thing in the first place. There was never any "goal" for the dwarves of your fortress, other than to survive, and by the time you're anywhere near the endgame you've probably reached a point where they can produce more than enough of whatever they need, and have found some way of dealing with sieges rather consistently. If not, your endgame is likely to be starvation, or genocide, and another scripted one isn't needed.
I've hit a lot of un-scripted endgames. They were all more exciting and appealing than the idea of digging into what I know to be rock that will start a timer that will unleash a baddy that I have prepared to defeat.
Assuming your fortress is pretty self-sufficient, you have absolutely no incentive to dig deeper, since you don't need whatever is there (and are, presumably, offing any noble who thinks otherwise). You probably create more wealth than you need to import, and certainly solved your food problems early on. The fact that you're continuing to develop your fortress (and play the game) is certainly more for the love of your fortress, and the joy of playing than any desire to reach the end of a game - so why have an end to the game at all?
Also, I don't like the idea of an endgame scenario because its boring. It shouldn't be determinable ahead of time, if there is one. Once you know that digging adamantine triggers the demons and the end of the game, you just avoid it if you don't want the game to end, and have all the time in the world to sufficiently prepare for it if you want to take them on. I like most of the suggestions involving different endgame sequences for different fortresses - but unless they are emergent from the simulation (like a plague or a battle that goes the wrong way) they would take way too much effort to be scripted in.
Half of the fun of playing and showing my friends (who also play) my fortress is exchanging stories about the things that happened in there - the unplanned stuff. The endgame story is about as interesting as the prelude - "seven dwarves show up next to a rock wall". Its the same for everyone. When MY endgame is starvation during a winter after monkeys stole all my food supplies... THAT is a great DF story to tell, and its interesting because it didn't happen to EVERYONE (well, at least not to all of their fortresses). If my farm is overrun with locusts, or a skeletal elephant manages to take out all of my woodcutters... those are the sorts of anecdotes that make DF fun for us.
Given that Toady is already working on it... there will be an endgame... so I'm overruled. :-D Since there has to be one, I agree with the posts above (including Toady's) that say that the "endgame" should be something that lures you in. Failing that, it could be something you're increasingly pressured into doing (and not by whiney soon-to-be ex-nobles). There should be some reason why you either want, or need, to dig that deep, or simply a happy-ending where you did whatever the dwarven people wanted you to do when they sent you out there - and that by choosing to end the game that way, you're avoiding some worse ending in the future.
It probably goes against the spirit of the game, but having some end-goal in sight would probably go a long way towards getting the user to the endgame. Maybe the REASON the dwarves were put there were to find adamantine to (somehow) save the princess who is going to die (from...adamantine-deficiency?) in like... 4 years, or 10 years, or whatever. There, now they have a reason to hurry their little butts into that mountain and find the adamantine. Making the fortress and all of the infrastructure necessary to sustain it, would simply be a way of insuring that they don't die before they find it.
Personally, I don't like that, because the fortress and all of that infrastructure is most of the fun of the game. Having some King on my back about digging out some cursed rock to save his spoiled brat just makes me want to open the floodgate to his throne-room.
It would be rather neat if there was a "good" endgame condition. Like, after you reach a certain amount of dwarves, with a certain history of production (exports > imports, food supplies always going up, etc) the fortress no longer needs your oversight and becomes a dwarven city in the persistant world just like any other civilization. Instead, you take seven dwarves, a wagon, a mule, and a few supplies and march off to establish the next settlement - but now you've added something cool to the world, with your own personal touch. You could later trade with them, or go to war with them, or whatever.
It might also be difficult to reach a balance where you're avoiding one end-game scenario but also don't want to hit the other one. You don't want to be too self-suffient, or the dwarven king will take over your well-established stronghold, but you don't want to be too poor or weak, either, or you'll die to the next siege or starve from one bad season of crops.
That way, most dwarven fortresses would end at some point, either with success (the establishment of a new town) or failure (they are all wiped out), but not by some plot-device. If you've got enough time on your hands that you're making doodles out of 1x1 bridges, the game will pleasently congratulate you on your victory and introduce you to your next challenge.
That might be a cool way to gradually increase the challenge. As more and more cities populate the world, they are fighting over land and other resources. Your 5th fortress might be in some haunted forest, or a glacier, or a desert - but by now the game is certain you've got what it takes to pull it off. Especially if you get to take some exceptional starting supplies from your previous fortresses.