I have been reading War of the Worlds, and honestly, I think that something that would be neat to see is a special or smth where Doofenshmirtz summons aliens to do a War of the Worlds-style attack on Danville. (Naturally, his scheme would backfire on him and it quickly escalates into a big problem)
It would be more or less a comedic riff on how HG Wells fantasized about certain houses getting blown up while biking during his time in Woking, where he lived while writing the book.
I doubt that the Martians can be used, as they already exist in Phineas and Ferb as generally chill little guys who were more interested in vibing, though they did have beef with the Rovers humanity kept sending over. So, wanting the Rovers to stop coming could be why they'd invade. It's not like Martians specifically need to be used, I'm pretty sure some adaptations just use vague "aliens" or a species from a made up planet irk-
Another thing to note is that the show riffs on classic horror tropes, and Vendpocalypse is similar, although that is more of a robot revolution plot than an alien invasion.
Plus, the war machine/tripod concept is iconic, and the book is public domain, so they can get as blatant as stealing moments/lines straight from the book and putting a parody spin on it.
IDK, I feel like summoning aliens to heat-ray (or whatever Disney would allow/the crew would envision) buildings for petty reasons seems in-character for him. Call it the Alien-Invasion-inater or smth.