Hello!
Okay, right now, I can't check this on the computer I have DF installed on, so I can't verify whether this is possible and works as intended - but if it does, it might be the kind of challenge you had mind:
Gen a world where all dwarven civs got extinct (if you are interested, I can help searching for that).
Unless I am mistaken, even if the dwarven civs are extinct, you should be able to play fortress mode, using dwarves from one of the former civs.
Again, unless I am mistaken, you shouldn't see any regular immigration under such circumstances.
If the above things work as I think they do, you could then play this as the "Return of the Dwarves" challenge. As your 7 are the sole survivors of the dwarven race, you need to bolster up their defenses and turn the fortress into the source of the new dwarven civilization with a population of over 100 dwarves.
If you want it to be really nasty, choose a site with goblins nearby (uppermost slot). As you will take a long time to reach siege size, you will be constantly attacked by ambushers.
I am sorry for being so vague about whether this is actually possible without modding, but I wanted to share this idea and it will be a while before I can check it (and the wiki has no real information on this).
Deathworks