I'd like a spontaneous magic system. However, I'm well aware what that'd do to balance, so specific spell slots it is. I do think that the game should offer a little more room for style, but that'd be served by little blurbs to the effect of "feel free to alter the spell descriptions and minor effects, so long as things like range, duration, and damage are never increased by said modifiers". As is, the thematic mage or sorcerer is a little underpowered, but themes are still primarily the province of role-play.
On the other hand, one of the big things that always bugged me about D&D was that most of the spells had no cost, and no chance of misfiring. Magic is supposed to be unknowable and uncontrollable, but spells are fired off with technological precision. I wouldn't mind a Vancian style wizard or a core style sorcerer, but at least give their casting level a little tweak in some direction or other (the old wild magic level variation table, maybe?), and some sort of spellcraft-style roll to see if the spell gets off as well. Probably balanced with more casting slots, and the ability to take temp. Con damage for a couple more slots above and beyond that, but basically a way to make magic a little less knowable and predictable, along every step of use.