For some reason, watching the 7th Harry Potter movie got me to thinking about shapechanging magic in D&D (despite the fact there is no shapechanging in that one).
In the Harry Potter universe, at least, magic that changes a living thing is difficult, with magic changing oneself requiring a good amount of time to master, and even then, you're typically stuck with one alternate form.
So I was thinking why not have D&D Polymorphing work similarly? The Polymorph subschool spells that each allow a single form are pretty well balanced. I got to thinking that even Shapechange is comparably balanced (it's extreme flexibility makes sense for a 9th level spell), just a couple minor tweaks (like making it never grant spellcasting) and it should be good. Nothing that you can obtain via Shapechange is out of the reach of a 17+ level character anyway.
My thoughts on the matter:
1. Either ban or alter Polymorph so it is risky to use, something like a low-to-mid chance for mishaps which result in penalties, half-changes, or even getting stuck in that form. With flexibility and lack of practice with a particular form comes risk of an improper change.
2. Use the existing Polymorph Subschool spells that allow a single form as guidelines to create more, similar spells for use. These spells may even have to be developed in-game with the mage desiring shapechanging magic to research it themselves, possibly giving them an ability nobody else has (which is kinda neat for the players, but also allows for the DM to say exactly what is possible).
3. Alter Shapechange so that it cannot be learned/cast without knowledge of some Polymorph spells beforehand. Also, give it a note about never granting spellcasting.
I'm strongly considering implementing these changes in my future games. Can anyone forsee problems with this or have any objections? I feel that drastically altering your form (going from, say, a Human to a Naga) should be much more difficult than granting your buddy some more strength.