I was thinking something along the lines of the Spell Point variant in the DMG, but with the base Spell Points Per Level lower than in the table on p289 and adding CON modifier to it. There could be an ability that allows the Sorcerer, in moments of desperation, to convert HP to SP (like how they can currently convert Sorcery Point to Spell Slots). Maybe even combine Sorcery Points and Spell Points into one pool so there wouldn't be a need to keep track of 2 separate resource pools. Then spells can be modified with Metamagic by spending more Spell Points. The exact numbers would probably need to be tweaked, but you get the idea.
Then, if you want to make Sorcerers MAD, perhaps have their Spell Save DC and Spell Attack Modifier use their CHA modifier. There's the seed of an idea here; maybe I'll give it some more thought when I hop in the shower today.
I think that's sufficient enough to make them NotWizards.
Wizards = learning spells from books = Intelligence
Warlocks = beg for spells = Charisma
Sorcerers = innate spell power = Constitution