Are you going to make this a Harry/ Daphne story?
My recommendation for Harry's animagus form is a Coatl. A snake with wings that can change into a any common animal. Coatls have the amazing ability to change on the outside but not the inner personality. I would also think that with Harry ability with occlumancy and putting up 2 minds having one form that can change into 2 completely different forms would reflect that. And naturally Harry also gets the form because he speaks Parceltongue.
“Fantastic Beasts and Where to Find Them.”
Coatl. These highly intelligent and magical creatures were once quite common in all temperate and tropical areas of the world. They have many magical talents, including shapeshifting, invisibility, invulnerability to fire and a variety of powerful psychic abilities. When their fangs and scales were found to be quite powerful potions ingredients, the Coatl proceeded to change themselves into non-magical animals and live as if they were not in fact, Coatl simply to protect their species. Their bite is highly toxic, and they can travel long distances magically in a way quite similar to that of Phoenixes and house elves. A name for this method of travel has yet to be decided upon.The number of Coatl in the world is completely a mystery as one has not been sighted in its natural form in several centuries.”
If you deem that to much of a magical animal, then a bird some kind. And as for Daphne then a black panther. Not very unique I know but we don't really know this Daphne very well.