I've been playing fortresses with this mod for the past week or so, and it's been an immeasurable improvement over the basic game. I played a lot of Orcs and a little Relentless Assault prior to df2010 dropping, so had an inkling of what I was getting into, but ... whoo, boy! My most recent defeat was to a simultaneous siege of naga, frogmen riding Giant Centipedes (?!), and war elephants. I think we killed 2 elephants in the time it took them to kill more than half of my military. D:
The one thing I miss is that the Orcs mod (or at least, the version I played) had Orcs active all year ... depending on how quickly I geared up, I would typically face a siege in Autumn or Winter of the first year. It seems that it takes entities much longer to cross the world map, now, and I'm having a hard time triggering Stranglers in the first year -- hampered even more by the fact that they (nor does anything, seemingly) appear in the winter.
I think keeping civilizations to three-active-season schedules is a good idea, but I think that they should be staggered in a way that creates a better arc. In every one of my fortresses, aside from one or two times when I would get a single civ (usually frogs) sieging more than once in a year, I would simply get a single siege by multiple civilizations (3-4) in the last few weeks of a season. "Winter is completely safe" is also a bit predictable, (can goblins show up in Winter? I haven't checked the basic raw), and something as simple as the bottom-tier civilizations showing up in Winter to harass your outdoors dwarves would add an extra element of guard.
Also, I'm asking for balance considerations -- how skilled I can get my dwarfs in any given amount of time is out of my hands, to a point, due to the randomness of training choices and how little they'll practice their weapon skill until everything else is a year+ trained. Stranglers and Frogmen (at least) on a Summer/Autumn/Winter schedule with the lowest-possible population triggers should help decouple them from the second tier, and provide additional fodder for live combat training.