A good solution to the age-old problem with Polymorph Self or Other spells is simply to restrict morphs only to general humanoid types, normal animals or giant versions of normal animals.
This would reflect the magicless properties of the caster himself or something along the lines of not having any magical blood in themselves. Monsters, supernatural stuff and anything beyond "normal" creatures in our own real world simply would not happen. You could even retain your spellcasting ability provided you morph into a form that has adequate appendages and vocal cord capabilities (humanoid here). This helps to keep things in balance while still providing ample opportunities for creative spellcasters to use the spell to good benefit in various situations.
You could also allow spellcasters to morph into monsters and other creatures provided they have a material component of the actual creature itself (some hair, skin, tooth, bone, etc). All of a sudden morphing into a red dragon by Polymorph Self because more of a challenge than before and in itself could present the basis for an adventure. Searching for dragon scales anyone?