Ok, hear me out on this.
If anyone doesn't know the movie Spies in disguise, then I highly suggest it as I thoroughly enjoyed it personally.
Anyways, it's a movie about a Spy accidentally turning into a pigeon, and I think that's enough reason for this concept. But the Personalities of the characters would work wonders with the light-hearted and humorous tone of the show in my opinion.
Phineas and Ferb is something I believe most people know, but to put it simply, It's a show about two Genius Brothers making the most of their ridiculously long summer by breaking the laws of reality with their insane and fun inventions, all while their pet platypus fights a mad scientist due his secret job as a secret spy.
that last part is important for this idea, because Perry, the platypus, is part of a secret organization named the "organization without a cool acronym", or OWCA, where animals are trained to fight evil.
I think you can see where I'm going with this, right. well, I hope so, because I'll be spoiling both things a bit as I go over potential plots and reasons for the crossover.
here's the idea so far: In Danville, Doofenshmirtz "accidentally" creates a powerful death Ray in an attempt to get revenge for another one of his backstories, except he didn't mean to create a death Ray in the first place, instead trying to create an innator for some villainous plan, or maybe he built an innator running off of a powerful power source that makes the essentially harmless innator seem like a death weapon. Anyways, the theft of the power source is brought to the H.T.U.V., who send out Lance Sterling and Walter Beckett to stealthily observe and take the power source back, only to Face literal animal spies, a joke of a supervillain, and maybe Walter finds himself in the backyard of Phineas and Ferb, falling victim to one of their grand adventures, who knows, maybe the created an equally powerful power source, and the two spies split up to investigate the two similar power sources to find out which is the real one.
again: A stupid idea initially, but very intriguing with a lot of potential in my opinion.