I'm not fundamentally opposed to the idea of meta-magic, but I don't like the implementations I've seen:
- skill rolls, possibly with penalties for specific effects;
- feats, which enable preparing/memorizing modified versions of spells.
What should be modifiable are qualities instead of quantities. Take for example Protection from Evil, which comes in a personal and 10' radius version. Or Charm Person versus Charm Monster. Blink, Dimension Door, and Teleport. These are the kinds of variants I prefer in a spell system. And what I think should be modifiable by meta-magic are variable effects. For example, I could see modding Protection from Evil on the fly to protect against Good and Evil and unaligned (neutral) simultaneously -- basically, turning it into the AD&D Armor spell. Or modding the LBB Charm Person so that, instead of causing instant friendship, it causes "delayed friendship" with regards to a single instruction, basically eliminating the need for a separate Suggestion spell. Area effect attacks could be modded to be selective -- a Friendly Fireball spell that allows allies to avoid all damage on a successful save, instead of half the damage.
But how would I implement such mods, if not with skills or feats?
The original spells did not require material components at all, with a few obvious exceptions (Magic Jar.) But I've heard rumors of an old spell mod system based entirely on material components. I would let magic-users research or experiment with material components to create variant spell effects. There would be no need to prepare the variant in advance -- an MU with Charm Person memorized could either cast that spell or crush a certain gem after showing it to a target in order to transform the spell into a suggestion.
Some components would be obvious to the DM, while others could effectively be randomly determined. For "obvious", the random effects of Reincarnation could be narrowed to a specific type of creature by using a piece of a creature of the desired type; similarly, Invisible Stalker would be the generic extra-planar creature summoning spell, modifiable using other creature parts.
I'm sure I'll come back to this subject later to expand the idea.