What stats control

[Sadrik]

constitution is how far you can push yourself
strength is how much you can lift
being able to lift things doesnt affect your ability to fight off illness

And neither does how far you can push yourself.

I think the best explaination to warding off disease and sickness is how healthy you are, which could be described as being how fit you are and fitness as a term could be applied to both strength and constitution and possibly could be applied to strength even more than constitution. How would you describe a heavy set (fat) guy, strong? yeah somewhat but not like a fit person, their advantage would be in toughness, taking a hit, or constitution. They are not necissarily good at warding off disease though, in fact they may be more prone! So there I think that adequitly applies Strength to the Fortitude save.

I must have expressed myself poorly. I did not mean to suggest that a wizard's bonus spells would be governed by anything other than intelligence. I was counting the level up bonuses (+1 to an ability score at levels 4, 8, 12, 16 and 20) in my examples, though.

I went back re-read your post. I miss-read it. Sorry.

As for spell DCs, I'd rather link those to the power of the caster as opposed to the power of the spell. Having the DCs start at something like 5 + caster level + casting attribute modifier allows low-level spells to continue to be a threat even at high levels. Spellcasters would then have a reason to use their low-level spell slots on minor threats and their high-level spell slots on major threats. Granted, this could lead to a DC 30+ charm person being used on a dragon or something, but I'm cool with it since that sort of thing has precedent in myths and fantasy literature. At the very least, bards would be able to do something cool like lull a dragon to sleep by playing a song. In addition, feats like Spell Focus become a bit more useful.

I can see your point about the saves not matching up right. But zowey, this would be way too powerful. I think a better way to do this would be to simply say you can memorize any spell at a higher level. So that charm person if you memorized it as a 9th level spell its base DC would 19 instead of 11.