Outside of letting adventurers recreate Shadow of the Colossus, however, what exactly is the point of a passive or neutral Forgotten Beast? Titans being more varied would be fine, but Forgotten Beasts in particular are basically just meant to be "boss fights" for fortresses against powerful enemies when your fortress gets large and rich enough to be presumed to be ready for a big battle. Adventurers rarely find Forgotten Beasts, as the caverns are a nightmare to navigate and they're not as prominent in rumors as the Titans, as well. You basically only run into FBs because they want to kill you, so having FBs that don't want to kill you might as well be an invisible teapot in far orbit around the sun.
Also, currently, FBs don't eat at all. I mean, it would be too easy if you could just starve a FB out. Even if they are like goblins, and don't need to eat, but do it anyway "for funsies", how would this be relevant to the game? What stories would be created from FBs randomly eating different types of things? Players need to have a way to interact with the eating of an FB to make it relevant. Even if FBs like eating dwarves, unless they have to actually stop fighting to chew when they get a kill, it's not likely to change too much about the game if they eat dwarves. Maybe if they ate metal or something, and they're invading the fortress to get to metal deposits or can eat axes right from the hands of dwarves, like a rust monster does, that would be an interesting change, and explain why it's coming after your metal-collecting fortress.