What if each level had its own AoE damage spell (except 3rd by tradition has two: Fireball and Lightning Bolt) but each of those spells does a different type of damage. If for example you want AoE cold damage you have to use Ice Storm, a 4th level spell; at 5th level there could be an AoE acid damage spell, at 6th an AoE radiant/necrotic damage spell (depending on caster's alignment), and so on.
I don't get this - are you saying a 10th level Wizard shouldn't be able to cast 1st level spells any more? If not, what are you saying?