I don't have time to try and write up any kind of a story about this particular idea.  But, I'd still like to have it writen down so I don't forget it.  Maybe I'll come up with something for it at a later date.

So, My thought is yet another Magic TG idea.  It starts with a married couple.  They both love each other, but they both have certain physical things about themselves they wish they could change... aka breast and penis size.  They come across a spell that allows them to switch bodies.  This spell is set for a certain time frame and will wear off as soon as the time is up.  Durring this time, every load of cum the girl swallows from oral sex will increase the girls breast size.  And every load of cum taken in her pussy will increase the mans penis size. 

The catch for this spell... if the girl gets pregnant durring the course of the spell, then the switch becomes permanent.  So the guy, wanting to increase his penis size, will have to take multiple loads of cum into his pussy, each time risking pregnancy.  Giving this spell something of a russian roulette feel to it.  Birth control will negate the effects of the spell.  If I happen to get worried about the "permanency" of this stipulation, then I could add a stipulation to allow a swap back.  I haven't incorporated anal sex yet...