I've never done a d20 system house rule before, so I'm not sure how this system might be improved, or how a clever player might break it. Here's the idea:
* There are no "spells" or "spell slots."
* A magical effect is tied to a particular Knowledge sub-skill. Blowing down the gates of a castle, for example, would be Knowledge: Architecture.
* To see whether you know the mystical principles underlying this effect, make a Knowledge skill check.
* The desired effect is classified as Apprentice (DC 10), Journeyman (DC 15), Master (DC 20) or Grand Master (DC 25).
* Magic would generally require gestures, speeches, candles, pentagrams, voodoo connections, etc.
* Magic would also come with certain "laws" - nothing longer than an hour, gotta touch the target, etc.
* Circumventing these restrictions is a matter for Spellcraft - tweaking your knowledge to achieve specialized effects, like longer duration, silent casting, etc.
* Casting is done with Concentration: maybe you know what to do, but doing it right is difficult. Concentration DC is probably 15 + level.
* If you miss your Concentration DC by 5 or more, there's a freaky mishap.
* I guess you can Take 20 on the Concentration check by doing everything juuuuuust right, but it takes a long time. Think Gandalf at the gates of Moria.
Under this system:
* Wizards can improvise spells, rather than using slots, but will probably take longer after much thought.
* Meta-Magic feats go away, replaced by Spellcraft and the feats which boost it.
* Item Creation feats might get folded into Spellcraft & Use Magic Device.
* Anyone with the requisite skills can bumble through a spell or two.
Still to be Resolved:
* I'd like to throw Decipher Script in there for something useful.
* Exactly what types of effect rank as Apprentice, Journeyman, Master, Grandmaster
* Exactly what's possible using Spellcraft
* Role of Druids, Clerics, Adepts in this system.
* Could Experts with the specific skills cast spells too? I'd like to say no...