I wouldn't want to morph anything huge(like an elephant), because they would be WAY too noticeable. I wouldn't want to have no place to hide.
Nor I would morph anything without inner skeleton, like slugs, snails, insects and insect-like organisms, any loose aquatic animal(such as jellyfishes, squids, etc.) and so forth. I wouldn't want to morph reptiles/amphibians or fish, either. Especially not plants or bacteria, if those even are morphable. No way...