They'd need to combine more spells, like they did with Bigby's hand. Or even completely rework spells altogether, so that you learn the "throw fire" spell, so with a 1st-level slot, it acts like burning hands; with a 2nd-level slot, it acts like Aganazzar's scorcher; with a 3rd-level slot it acts like fireball, and so on. This would make a lot of sense, lore-wise, since lots of fantasy fiction assumes that their spellcasters progress in a manner like this. You could probably cut the spell list in half or more by doing this.
And/or they could go back to "chance to learn spell," thus ensuring that not everyone will end up with the spell list.