I have a system of Metamagic that keeps low-level spells attractive at higher levels. Basically, the idea is that you can metamagic any spell up to the level of the highest level spell you know, thus keeping low-level spells viable and reducing the 15-minute-mage problem.
I'm posting it here, and hope it makes some sort of sense:
Forget the original metamagic rules except when creating magic items. Applying metamagic is a free action for all classes, and it is never prepared in advance. You can apply metamagic that brings the effective spell level of a spell up to the highest spell level you can cast, for free. You can apply the same metamagic feat several times to the same spell, with cumulative effects.
Example: A lvl 9 wizard can prepare 5th level spells. This means he can apply 5 levels of metamagic to his level 0 spells, 4 levels to his level 1 spells, 3 levels to his level 2 spells, 2 levels to his level 3 spells, and 1 level to his level 5 spells. This does not change the spell slot used.
Add to this my modified version of Heighten spell:
Heighten Spell (Metamagic)
Benefit: A heightened spell has a higher spell level than normal (up to a maximum of 9th level). Unlike other metamagic feats, Heighten Spell actually increases the effective level of the spell that it modifies. All effects dependent on spell level (such as saving throw DCs and ability to penetrate a lesser globe of invulnerability) are calculated according to the heightened level. The heightened spell is as difficult to prepare and cast as a spell of its effective level.
If a spell is heightened by two levels or more, all of these additional benefits apply. Each applies once per two levels of spell level increase. I.e., a first level spell heightened to sixth level (+5 levels) gets each of these bonuses twice.
* Increase the cap on the maximum amount of damage your spell can do by five dice.
* Increase the limit on any spellcasting level based bonus in the spell by ten.
* Affect four additional hit dice of enemies. Only applies to spells who affect a limited number of hit dice.
Example: Tim the Enchanter is a ninth level wizard who considers how he can prepare various heightened spells. Hypnotic pattern is normally a first level spell that affects 2d4 hit dice of creatures, plus a number of hit dice equal to the caster level. Tim can prepare it in a third level spell slot, and it will affect 2d4+13 hit dice of creatures, or use a fifth level spell slot to affect 2d4+17 hit dice of creatures. A fireball that uses a level 5 spell slot has a damage cap of 15 dice, while a seventh level slot gives it a cap of 20d6. A dispel magic that uses a level 5 spell slot has a cap on the dispel check of +20.
They system I've used with success is - you metamagic on-the-fly as a free action any spell of any level but only 3 times per day (2/day for maximize, 1/day for quicken). It's pretty much a modification of something Jim Butler of Bastion Press came up with years ago, that Monte Cook also created and published in his Book of Experimental Might.
Before using this modification, no PC in any of my campaigns ever took a metamagic feat unless it was a prereq for a prestige class. Afterwards, they're being used some, but not overwhelmingly - about on par with any other group of feats. So, in my campaigns at least, this change made metamagic feats actually desirable. Plus it also makes them much more simple than all this determining what level you can and can't for each individual metamagic effect.
Considering you are spending feats on them, and it's limited per day, it hasn't made spellcasters substantially more powerful than they were before (and in my experience they haven't been massively unbalanced to start with).
The problem with high power wizards is not that they have many low-level spells, because those quickly become obsolete. The problem is that they have a few high-level spells of awesome power. When those are gone, they'll want to rest - thus the 15 minute working day.
What this tries to do is raise the value of low-level spells, thus increasing the wizard's durability without vastly increasing their powers. It also makes metamagic feats viable.
I've used it for a year now, but as I also used recharge magic, its dangerous to draw any conclusions from this play test.