In my campaigns, sorcerers can learn metamagic-affected spells as Spells Known, without needing the metamagic feat in question, and without increasing casting time. Alternatively, they can take the metamagic feat, and apply it to any spell they want.
So, for instance, a sorcerer could choose Empowered Ray of Enfeeblement as a third level spell known. They don't need to have the Empower Spell feat, nor do they need to have Ray of Enfeeblement as a first level spell. But they also can't apply any metamagic reducers, etc., to make it anything other than third level slot. Alternatively, a sorcerer could take the Empower Spell feat, and apply it to any of his spells known as usual.
Regarding limiting spells known, I once played in a group that eliminated sorcerers and wizards altogether, and had one generic mage class. This class had a casting mechanic similar to the Spirit Shaman, where spells were cast spontaneously, but you could choose your "spells known" every day (with some restrictions). You could be a Learned Mage, which based your casting off of Intelligence, and gave you more flexibility in choosing spells, and eventually the option to change your spells known list with an hour of study, or you could be a Natural Mage, which based your casting off of Charisma, and gave you various "magic-blooded" abilities that were level based, and included things like naturally detecting magic, and eventually Spell Resistance.
I didn't play a mage, but from the two people in the group who did, it seemed like a pretty reasonable alternative to sorcerers and wizards. However, I don't recall what restrictions they placed on them. I do know that the end result didn't necessarily balance the game as a whole, but it did do an excellent job of keeping the wizard and sorcerer on the same tier.
