If you start in range of the goblins and get them to siege, wouldn't casualties be subtracted from their populations? A fortress focused on churning out high-value items and elite goblinsmashers might also help the home civ by depleting the opponent's numbers? Or doesn't it work like that (yet)?
Each siege contains very few historical figures, so the slaughter really doesn't do much on the historical level. As far as I can tell only the leader is a historical figure, with goblins who managed to kill a figure will also becoming one.
I would use StrawBarrel's logic to seed the world with fortresses out of reach of the goblins (which does not have to be on a different land mass, if the world is large enough and the goblins haven't covered it all) to build up strength, coupled with large breeding programs. The breeding is needed because most (or possibly all) of the migrants to subsequent fortresses would come from previous ones.
I've found a small continent with just elves and humans that I'm going to settle. Nīrumid, or Landfall, is the site where the rebirth of the Gears of God will begin.
I like it. It has a reverse Shining Force 2 feel to it. We were on the big continent and fled to the small one. I feel like at some point I need to make a large fortress called Groundseal.
I intend to cap the number of dwarves at a somewhat low level, both to try to prevent draining previous sites, and to have room for births before FPS drags too low.
I'm not sure how the breeding program will go. As noted in other threads, getting relationships established is complicated, and my last project at Futureswords owes it partial success solely to migrant couples. Friendships did develop, at least. I made no temple, no tavern, no nothing, so dwarves would socialize in the dining room instead of wasting all their time praying and meditating. There wasn't that much time available for idling though, what with all the building projects and the mandatory military training one month out of 6.
I think it could have worked given more time, but the flying glass titan that spawned too close to the fort to get locked out ruined those plans. Lesson learned, don't build near the edge.