The only thing I don't like about taking the ability modifier out of the equation is that the game's "balance" is built assuming that the ability modifier is in there. One of the effects of all the buffing spells and buffing items is that it will alter the save DCs of spells and other effects, and this is part of their cost, part of the save progression, etc.
It's assumed that, as characters advance in level, their ability scores will have these quirky jumps, but that they're factored in to the whole system.
I do like the idea that a higher-level caster will have a harder-to-resist spell though, so I'd like some variation that fits with everything.
10 + spell level + one-third caster level + one-half ability modifier
The problem is, this gets unwieldy. Another unwieldy option (but at least one with a quick reference) would be this:
10 + spell level + twice total # of bonus spells
If you have a 16 Int and are 4th level, you get 2 bonus spells (a 1st and a 2nd). If you have a 20 Int and are 14th level, you get 6 bonus spells. In these cases, your save DCs would be 14 + spell level and 22 + spell level, respectively.
Dave