"What Might Have Been" is a duet between Phineas and Isabella in the Season 4 Phineas and Ferb episode "Act Your Age". As Isabella is leaving for college, she reminisces about all the past summers she spent with Phineas. Meanwhile, as Phineas is walking down the street, he thinks about how oblivious he was to Isabella's romantic advances over the years, even though in hindsight they were really obvious. As they go off in two separate directions, they both wonder what might have been if they had gotten into a romantic relationship together.
Isabella: I wanted you to see me, But for so long, you were blind Now it's time to face tomorrow, And leave all that stuff behind Of what would our lives have been like If you'd just given me a sign 'Cause I like you more than every other Thing I like combined
I spent so many summers Hoping something would begin I thought that I was over you, But here I am again What might have been
Phineas: I can't believe that all this time, You never said a word Although it's possible you did And I just never heard I never even noticed I guess I wasn't that alert But I must say that, in retrospect, You were being quite overt
And now our endless summer Is finally coming to an end I tried to make the most of every day, But now the years just seem misspent What might have been
Isabella: I could've been your girlfriend Phineas: I could've been your fella We might've been an item Isabella: They would've called us "Phinabella" Phineas: I would've held the door for you I would've shared my umbrella Isabella: You could've held my hair back, When I was sick with salmonella (Other Isabellas and Buford Cupid: Sick with salmonella...)
Isabella and Phineas: We could've been together All you had to say was when And though I wanted so much more, I guess you'll always be my friend What might have been