I've tweaked the world again a bit, getting a better site density in the center area (just gives you more chances at a good fortress spot).
But the thing that I'm really working on, to make this special, is to have goblins with both trolls and beak dogs.
My process goes like this:
1. Start a world with the correct init profile.
2. Check that both the swamp and mountains in the goblin spokes are evil, and that the goblins have in fact settled there (goblins seem to like the dwarven lands too). Check that no other subregions are evil, and that elves and humans are present. If failed, restart.
3. Load up world and go to the goblin tower's region. If you can see the bottomless pit and chasm clearly don't intersect the goblin area, restart.
4. Embark on the goblin tower. Check to see if the chasm or bottomless pit is present, and that the goblins are hostile. If either condition fails, restart.
5. Abandon and use the site finder to find an interesting site- underground river, sand, ice, forest, gabbro, magma pipe, HFS. If not found, restart.
6. Embark on site and use reveal to check for bauxite and magnetite. (Optional: use a utility to check for giant cave spiders.) If not found, restart.
The closest I've gotten is step 3. And that was a lot of work! (And I probably won't use the site myself, because I want to use some modded materials in my next fortress- guess I'll do it twice!)
Now: can anybody confirm this from experience- that the tower itself must include an evil swamp block (for beak dogs), and that the tower itself must include a chasm or bottomless pit block (for trolls)?