Sorcerer as a prestige class

[Reddish Mage]

So... I accept that Sorcerer is the weaker class at high levels of play, but you do seem to be completely ignoring the adantages of spontaneous casting.

I don't think anyone is ignoring the advantages of spontaneous casting. However, there are RAW ways for Wizards to acquire spontaneous casting to some extent or another without losing the tremendous versatility and quicker access to higher level spells. It is also possible to cheese spontaneous casting technically by plane shifting to a fast time plane. All in all however, even you agree that the wizard is simply the better caster "at high level" (though really we should add at almost any level).

I'm going to take this in kind of a roundabout way, so bear with me.

Fluff first: the difference between Wizards and Sorcerers. The way I explain it to new players is that Wizards study magic, but Sorcerers are magic.

Leave aside (just for a moment) the fact that any player can choose to take a level in Sorcerer: from the fluff, if you don't have that magic in you to begin with, you can't be a Sorcerer. You can wield magic all you want, but you'll never be part of it in the same way a Sorcerer would be. So if you have a Wizard who's spent their whole life studying magic, and loves it to death, but can never actually be it... yeah, I can see how that motivation would work. It probably wouldn't be every Wizard, but it could apply to a good chunk of them. A Prestige Class focused on that motivation - putting mechanics to go along with the story - would definitely be something I could get behind.

what could it look like? Well, my first thought is that you'll have to seriously alter your ideas about class balance to even attempt this. You're already dealing with one of the Big Three. Wizard can already break the world in half 75 different ways, so I wouldn't be too concerned about giving it yet another tool to do so. You do have to make the benefits important enough that someone would want to take a level in it, rather than another level of Wizard (or another full-casting PrC). On the other hand you don't want to make the PrC so powerful everyone will want it.

So, maybe something like this.

Losing a single level of spellcasting for really nice goodies as well as the ability to cast any spell known spontaneously This is one seriously overpowered prestige class, especially if one can continue to take it past 5th level. The metamagic goodies are also better than the existing prestige classes.