Increasing the standard difficulty would only result in people complaining that it's too hard.
Difficulty POTENTIAL is the most important factor for DF, I think.
I think that is a very good way to put it. As I said, I like that the game can be easy, and I agree that the game should not just unilaterally made harder. I like being able to build my fortresses in relative peace before having alot to worry about. However the problem with the difficulty potential right now is that its not the game itself making things hard, its people making it hard by playing the game with self imposed handicaps.
I know army arc some things will get better. However, we shouldn't all just say, "Well the army arc will make things more difficult." The only difficulty in the game should not just come from war and trade. If that was the only "challenge" in the game, I would find it boring. There are plenty of other ways in which the game could be "potentially" harder. Some of them I am sure Toady already has planned or is even working on. However, the far end of the spectrum can definitely be kicked up a notch, and that should be acknowledged.
The biggest difficulty problem (in my opinion), is that the difficulty is tied so much to the site/embark options. There is not enough of a difficulty growth from starting a site, to several years in its future. A player should be able to start a fortress on a good site with a good selection of equipment without the worry that the fortress will not offer enough of a challenge.
With the idea of "playing the world not the fortress" perhaps you can then look at the difficulty curve as being between sites and not so much within a specific fortress. By that I mean you could look at each fortress as a plateau. Ideally the difficulty curve would perhaps then work with a player graduating to more and more difficult fortresses. However, if that is the case, there still needs to be something to kill off the easy fortresses, or some way to "graduate" or complete them.
I also recognize that there is alot of fun coming up with your own challenges. I love making huge burial halls multiple Z-levels high and elaborate water and magma systems as much as the next person. However, if the fortress does not have many other concerns, the construction projects usually just become a matter of time. I often find my biggest challenge in completing these things is the dropping FPS as more dwarfs come (I don't like disposing of immigrants, heresy I know. Though once I get my good computer fixed the frame rate won't be as bad).
The game DOES have difficulty issues. Claiming that it doesn't, or just trotting out "It's as hard as you make it," is nipping a potentially constructive conversation in the bud. That's not to say that it is not a good thing that players can make the game harder. There should be a spectrum of difficulty and its a good thing that the players have some control over it. However, it is a problem if the way to move up in the difficulty spectrum is to self handicap. Some ways of making the game harder, may have ultimately the same effect as some forms of self handicapping, but by making it a build in part of the game, the feel will be significantly different.
As I was witting this Lode posted. On his second suggestion, perhaps the different dwarven civilizations may differ in aggression. Allready some of them differ in who they are at war at, and they may already have some aggression tag; I don't really know. But this could carry over to your dwarfs in Fortress mode. Maybe random immigrants may also be more aggressive, or perhaps adventurous, giving you a legit reason to be concerned about immigrants. It would be interesting if dwarfs could choose to "explore" in their free time as a job. They would wander around, perhaps putting themselves in danger. Eventually when we can send out armies into the world, perhaps they could even become adventures (if their personality was so inclined) and come back with riches and tales of adventures, or return wounded and with the bad news that they have angered some sleeping mega-beast that is now following his trail back to your fortress.