From what I know, forgotten beasts and titans, if you happen to receive an organic one, such as an eyeless eightleggged quadruped, and butcher it, you typically get quite a few products, such as meat, shell, bones, etc, but in particular, the webs. This would make web collection a bit more worthwhile, as right now, web spinning, colossal mites produce However, it is all equivalent to domestic animals, and after the effort of killing the thing, the bones and such aren't worth much. I propose that having some sort of random value modifier might be added, perhaps proportional to the difficulty. Nothing ridiculous, possibly ranging from 1-4. This difference would make forgotten beast and titan encounters, just a bit more worthwhile for some players, myself included.
Clowns could be a possibility, particularly web spinning clowns. Depending on how high, might cause an outbreak among players of farming the circus, either way, would be a lot of !!fun!!.
A tidbit and thing to ponder: perhaps inorganic forgotten beasts should drop a couple of rough gems, if they are made of a form of glass, amber, gems, etc. Ash blobs and such, there wouldn't be too much stuff of value. It would definitely give you a reason to kill that green diamond forgotten beast you have trapped, in the name of profit. Another possiblity might be statues made of the material, in an image of the forgotten beast or titan, with all the battle injuries occured. Example could be "This is a masterfully crafted amber statue of Nolthag the three eyed six legged dimetrodon crafted by an unknown artisan. Nolthag is slavering. This statue depicts the wandering of Nolthag in the caverns." Perhaps something similar for bronze colossi and iron men, though it is probably in progress already. For the quality, instead of having always masterwork, maybe the damage dealt can be lowered based on how much damage it has sustained. So if you chip at it all day but don't kill it, it might be reduced to superior, and so on. Maybe if there is sufficient damage dealt, it might drop several rough gem boulders instead of a statue, but hey, whatever you have to do to get it killed.
Finally, perhaps there would be some sort of value added to your fortress for stuff slain. Megabeasts, titans, goblin generals and such would count primarily. Cats and regular goblins wouldn't add anything. It would be an interesting version of fame. Not necessarily wealth, it might attract more immigrants in general, or millitary dwarves, looking for fame.