Hi!
I rarely have large fortresses. However, when the big immigration waves hit, I have the following thoughts:
* Unskilled peasants are not even replacement labour. So, I make sure that each peasant gets some proper job where they can quickly get experience to improve both their professional skill (in case they are needed to replace a dead dwarf) as well as their stats (if I do decide I need to draft them). I never draft unskilled dwarves (except for when an unskilled dwarf discovers a kobold thief on the tile she is standing
).
* Engrave a lot. I usually do a lot of engraving. Usually, all workshop rooms are completely engraved. Community burial chambers are completely engraved, meeting halls are completely engraved. Over time, finished corridors get engraved (finished in the sense that no new digging will take place there).
* Remove legendaries from their favorite jobs. Especially with legendary engravers, I tend to remove the job they are legendary in so as to heighten their chance for increasing other skills. This also prevents them from taking away the work for unskilled workers.
* Traps. In my fortresses, a really complete room also has a stone fall trap at its front door. This can add quite a lot of work for your dwarves.
* Statues and decorations. Statues look nice in meeting halls, but they also add to community burial chambers. Even corridors can be improved with a few statues. Besides keeping your dwarves busy, it also raises the morale.
* Encrust/decorate. You can put a lot of decoration on statues and the like. Have them decorated until at least one interesting image is on an item. Then replace a non-decorated item with the newly decorated item. Repeat. Besides statues, doors are also good condidates for this.
* And the all-time favorite - complex building projects, like constructed guard towers with barracks on top and the like. If you really have more dwarves than you need, make all constructions with blocks only - this can keep mason's workshops and masons quite busy.
Well, I hope some may find these thoughts useful.
Deathworks