I wouldn't mind a Persona with a cast of characters around the age of Catherine's main cast. Part of the reason Catherine's story was so great was that its older cast allowed them to tackle a different set of issues, and I think it would be great to expand the series that way.
The problem is that it misses the point of Persona, in a way; if we consider the origins of Persona with "Shin Megami Tensei: If...", the point was to be Shin Megami Tensei, but in a high school setting. The whole SMT in high school part is necessary to be faithful to the original idea, really.
As much as I'd like them to, I can't imagine Persona 5 moving away from high school.