this will be great for underwater tile sets - shoals of fish
Could have fish and birds in tileset...?
Well, hypothetically. If a tileset object was replaced with a model of fish, it would technically work, but only on maps made specifically with that tileset in mind, otherwise the object would likely appear in places it shouldn't (such as fish on land). Birds would be a bit easier I guess, you could have a model have an animated bird appear as well (for example, one of the tree models could also have animated birds, which could be given a large animation to have them appear to fly around the map. However, if that tree appears too often, there'd appear to be way too many birds. Not to mention they'd be all over the place, all flying the same "strokes" (if in random rotations). Not sure if you'd be able to see the bird if the cell the object is on is outside of the camera.
So to answer your question, yes, it's possible, but wouldn't really work that well, so I wouldn't say this is the "solution".