Doing a experimental game in a world of high magic, magic is a irreversible part of this world, common peasants have access to magic, however, magicians, those who specialize in it's use, bond together in guilds to take up jobs that pay well, this world is full of mystery, magic, and more. The following changes would be made

ALL magicians of any kind have to take the Thematic Magic feat without meeting the perquisites, they must choose a form o spell casting "Alchemist, Wizard, Sorcerer" ect, and this ability is granted to a class such as rouge, warrior, in exchange for a lower hit die, and a lower BaB.

I guess this answers my interest check

Are you still doing this? Cause it sounds worth a shot to me. Got a couple questions though.
The Thematic Magic feat, is that the Spell Thematics feat from Faerun? (I'm thinking it's something else entirely from your post.)
Also, the setting sounds a bit like the Fairy Tail anime, coincidence?