Well, soon the 104 days of Phineas and Ferb vacation would end, of course they would extend the summer, or recreate another summer, but let´s think of how it would be if they end showing the characters going to school (and not only a spin-off, or the sad ending when everyone cries like toy story 3).
I think it would be cool to change the story a little, I don´t hate the repetitive formula (in fact I love it, because it´s just like real life when every day seems the same, but it´s different because the jokes with friends), but I think than characters like Candace just got stuck.
Candace needs to do more "teen things" than learn driving and her crush, maybe if they go to school, she would have more indepent subplots because she would be in a different grade (maybe if would be cool phineas and ferb skipping grandes and end in her class, but only in an episode and then they decide to stay at their own grade, because it´s more fun or something).
Also, Candace and Vannessa charing some classes would be cool to make them more friendly rivals (they would fight on who have the most anoying relative).
And finaly, maybe other crushes for Candace or Jeremy to prove their relationship (of course they would remain together because this is not a soup opera).
For Phineas and Ferb, they would still have fun even if they were on school, teaching kids than fun it´s on them and not something.