Then don't take the lower level versions.
If we redoing 3.5.
Nah, I want a level 1 fireball (Least Fireball), 2nd level Fireball (Lesser Fireball), 3rd level Firebal (as is Fireball), etc.
1st level (Least) Fireball has short range (not long), 15 foot area burst (not 20), etc and deals 2d6 fire.
2nd level (lesser) Fireball Med range, 15 ft area (not 20) and deals 4d6.
3rd level Fireball: Long range, 20 ft area, and deals 6d6 fire.
Those who only want Fireball wait till 5th level.
Greater Fireball already exists in 3.5 (delayed Fireball)
But the idea from 5th to add casting stat to damage is a neat idea.