Hi!
I am still a newbie whose playing style will probably keep me from ever becoming a grand master at the game, but I am still having fun.
Anyhow, if you want a really different challenge, my suggestion would be changing the population cap. I am currently playing with population lock 6 and it is really very different from your normal game play, even on the easy maps.
Of course you are unlikely to have trouble getting all the food you need, so the early game may be easier, but dealing with outside menaces and actually getting anything done can become quite difficult. (I am currently worried about one fortress that has just got their trade minister ... ) Picking which dwarf does what and when is much more of a problem then when you have dozens of dwarves to pick for each job.
As challenge targets, you can aim to get as many nobles into your fortress as possible. Judging from the wiki, you should be able to get a manager (requires 20 dwarves and there are more than 12 nobles you can get without a big population according to the wiki). Having a captain of the guard would probably require you to be very lucky. Of course, managing a fortress with more nobles than commoners may really be nasty ......
With marriage currently not existant (T_T) , this setup makes expansion quite difficult.
Deathworks