I'll float an idea past you that you might want to look at - especially since wizards are (in a literary fashion) more likely to dabble with things they don't quite understand and have problems with...

You could allow wizards to spontaneously apply metamagic to a spell (and without increasing the spell slot), but attempting to bend a prepared spell in a direction which it wasn't prepared for is risky - there is a 10% chance per metamagic level not applied of the spell backfiring - damaging spells ground zero on the caster, other spells fail and deal 1d6 per spell level (like a backfiring scroll spell).

Thus if the wizard is in a really bad way, knows Quicken, and wants to attempt to quicken that fireball on the fly, there is a 40% chance that it goes off at ground zero instead :)

I deliberately suggest the use of a flat percentage which isn't modified by anything. Anyone can do it, but if you are 5th level and attempt to silently, quicken a stilled maximised fireball you've got a 90% chance of wiping yourself out...

(I try not to be a naysayer, and to give positive ideas if I can!)

Cheers