They are a little young, but even so I think that would be TOTALLY AWESOME! i look for a sing that Phineas likes Isabella in every episode and I've seen a few possible ones. If Phineas does realize it though, they should end up together, because if they didn't I'd be really mad at the people who make the show.
I have a good way to do so. Like Doof builds a Non-obliviousinator and it hits phineas (and just so আপনি know the effects are permanent:)) and he realizes Isabella's Crush.
i agree with DaNerd but i think thats one of the things Isabella like with him. she should get another chance in Paris অথবা PnF should build like a dream পাঠ করা machine and then she has no choice! :)