Hr...
Splitting up the spells currently in the PHB into 20 levels instead of 9 means that you'd end up with (on really rough average) less than half as many spells per level. That means each time they're choosing new spells, they're doing so from a small list. Two casters choosing one each from two small lists will tend to generate more similarities than choosing two from one big list.
Which means that unless you drastically change the "spells known" mechanic, or add a whole lot more spells, you're probably going to tend to see a lot more similarity among casters of a given class. I'm not sure I like that.
Splitting up the spells currently in the PHB into 20 levels instead of 9 means that you'd end up with (on really rough average) less than half as many spells per level. That means each time they're choosing new spells, they're doing so from a small list. Two casters choosing one each from two small lists will tend to generate more similarities than choosing two from one big list.
Which means that unless you drastically change the "spells known" mechanic, or add a whole lot more spells, you're probably going to tend to see a lot more similarity among casters of a given class. I'm not sure I like that.