Well, I don't really feel like genning a world, finding an embark site and then uploading it.. However I suggest downloading Fortress Defense II (you can find it on the mod page) and seeing how long you can last against the onslaught. If its going too well start limiting yourself with rules.
For example, the rules I have for myself are:
1) No completely walling/drawbridging yourself in (you have to have a minimum of one three tile wide doorway to your fort, you can protect it with doors or floodgates, but not walls or drawbridges or moats or any other impassable defense.
2) No 'trap' traps (you can create traps using water/magma/cave ins/whatever, but no building traps conventionally to kill your enemies (you can use cage traps for hunting, but you have to take them down after you catch whatever you were going after.
3) No 'danger room', its an exploit after all so its not something you should get used to (as it may vanish someday) and it makes things too easy.
4) No magma as fuel (you can use it for traps or whatever else, but no magma forges or smelters of any kind, if you want to cook up some metal you need to use old fashioned charcoal/coke/coal).
I think that was it, I have a few more small rules here and there but those are the biggest and most challenging. It certainly adds a lot of danger and a very specific goal (real survival, and to see how long you can last) to strive for.
If you are looking for something more megaproject..y I cannot help you, I personally haven't really gotten into those much, but I'm sure somebody else can.
-MB