Well, we haave to consider the range of possibilities. In 2nd Edition, in the Forgotten Realm Netheril Supplement spells above 9th level could do many things, such as create an effect that would encompass a city, or a geographic region to alter climate. (The highest level spell was 13th level and was cast by the wizard Karsus to steal the power of Mystryl, goddess of magic. This worked briefly, as Karsus was unable to contain the power, and Netheril came to an end.)
I suspect that levels 21-30 will be epic in content, but very different from the Epic Level Handbook. Possibly, the spells at those levels might be reminiscent of either what is in Netheril or in Monte Cook's Arcana Evolved, which has 10th level spells. One of the weaknesses of 3rd Edition play was support for Epic Level characters.
Possibly spell level now specifies the MINIMUM level that a character can cast a spell. So, it may be that whether a 25th level spell is like a current 9th level spell or more on par with something from the Netheril supplement or Arcana Evolved, it will require a spell caster of 25th level or higher to cast it.