The other day there was a question on the Dwarf Mode board about whether new civs are ever generated, or if existing civs split. The answer was, of course, no, they don't, they are generated at worldgen and can only go extinct. But of course, this is silly. Real world governments have civil wars and secessionist movements and such reasonably often, and it occurred to me that this would be a perfect starting scenario.
You are a group of malcontent dwarves that have decided to break ties with the mountainhomes and start a new glorious dwarven empire. This might give you a good reason to start a fortress in, say, a terrifying glacier. Since you will, in all likelihood, be at war with your parent civ by default, it pays to make your fortress remote and inaccessible. This would tie in well with
another suggestion currently on the front page: fortress visibility. You could defiantly put your rebel flag on your large and prominent ramparts, get regular migrant waves (malcontent dwarves from all over that have heard of your rebellion), regular traders from civs that are willing to offend the mountainhomes, but get assaulted by the full might of the mountainhomes fairly quickly. Or you could set out in secret, and carve a hidden fortress for your underground resistance, and get only a few migrants, from members of your movement that couldn't make it in the original party, but few other migrants and traders, and less risk of dwarven invasion until you come out into the open or are revealed by your carelessness (maybe occasional dwarven scouts could enter the map to try to find your fortress, especially if you are making a name for yourself?).
I think this would be a good, plausible starting scenario for what many players want: a hard mode. A good excuse to embark in a remote and hostile environment, and a strong military opponent with a sound motivation to throw their best at a new fortress. Dwarves are the only race with steel, and would pose more of a threat than your typical goblin siege, and it would make more sense for the mountainhomes to do its best to crush a rebellion in the early stages than for a goblin civ to throw army after army into the meatgrinder that is a well set up fortress.
This would also tie in well with
another current suggestion, by making it plausible that your fortress is the central authority of an expansionistic new empire.
Dwarves would also make plausible digging enemies, if that is ever implemented. One of the main reasons not to implement it (other than pathfinding headaches) is that many players will be deeply offended if the game messes up the stuff they built. This would provide a good non init file way to opt out of digging enemies: just don't start a war with the dwarves.