Interesting... by skills based were you thinking like the Force from Star Wars RPG? A much more limited number of effects and skill for each one.
Absolutely not. Personally, I can't stand the D20 Force rules... They're simply more truoble than they're work. Blecch.
Or one magic skill, that you roll to see if your successful higher level spells have a hard DC to pull off.
That's more along the lines I was thinking of. Although...
Or one magic skill that works like perform, so you spend skill points to gain new spells or improve your general casting.
That's not a bad idea either.
My initial idea was to use something similar to the Craft skill rules for casting spells... The rules for spellcasting in Spycraft's old Shadowforce Archer campaign setting worked something like that:
Each spell has a base DC, a base "cost", and a casting time. The casting time is how often you get to attempt the skill check, if you don't complete the spell witht the first check. The "cost" is the total amount of "mana" (or whatever you want to call it) you have to build up in order for the spell to begin taking effect. The base DC is what you have to beat with your skill check to add something to the "mana" total, with the amount added being based on the difference between the DC and the check... If you fail by more or less you make no progress, and if you fail by 5 or more something bad happens, just like Craft.
So, let's say that anequivalent 1st level spell might have a DC 11, a Mana Cost of 100, and a casting time of 1 round.
If the caster has a Spellcraft of, say, +6, then his casting might look something like: First round, Spellcraft check 14; 14 - 11 = 3; 3 x 11 = 33 mana points. Second round, Spellcraft check 12; 12 - 11 = 1; 1 x 11 = 11; 33 + 11 = 44 mana points. Third round, Spellcraft check 20; 20 - 11 = 9; 9 x 11 = 99; 44 + 99 = 143 mana points, and the spell takes effect at the end of the round.
If the caster is much more powerful, and has a Spellcraft skill of +15, he'll be able to cast the spell much more quickly, and have far less worries about making a dangerous mistake: First round, Spellcraft check 29; 29 - 11 = 18; 18 x 11 = 198 mana points, and the spell gets cast.
So more powerful casters have an easier time casting spells, and can cast simpler spells faster than less experienced casters. Also, this eliminates the need to categorize spells into spell levels.