I'd rather see that as an add-on or selection, such as the Madness Magic from the 3.0 AEG book "Magic" which does that.
Really, this is why I don't like classes. I want spontaneous casting, but I want a system where you can do other things when selecting magic style. You might have a choice between preparation and spontaneous, arcane and divine, int/wis/cha, materials and foci variances, formal and superstitious, scientific and mystical, etc. Each could have it's own effect on your magic casting before you even get to spell selection.
I.E. prepared casting has the advantage of more spells known, while spontaneous casting can have faster casting.
I suggest getting Spheres of Power, since you pick those type of things independently from class with it, and then get to pick what type of things you magic can do.