Warning: The following contains a fan-made theory, if you don't like this stuff, or for some reason don't like the Fairly Odd Parents/Danny Phantom, just stop reading now.
We all know about Timmy Turner, main character of the Fairly Odd Parents, always accompanied by Cosmo and Wanda, then we also know Danny Fenton, half ghost, fighting ghosts just about every day of the week. These two characters, even though they seem nothing alike, have a lot alike. First their hair looks about the same, next, their parents are always occupied with other stuff most of the time to pay even the slightest attention to their son.
I do believe that in fact, Timmy did use Cosmo and Wanda, he must have had at least one last wish with long term effects, and I have a feeling, that wish was to become "A new child".
He knew his time with them was almost up, and he didn't want to face the world alone, he was to scared, unprepared, he was just turning 13, so, with the last of his strength, and love for Cosmo and Wanda, he wished to become a different child, whom they called Danny Fenton, but it didn't stop there, Maddie and Jack Fenton have the same hair color as Timmy's mom and dad, they just resemble how he remembered them.
The Ghost Zone is used to hold a bunch of creatures with amazing powers, doesn't that sound at all like Fairy World? That's because it's how Danny/Timmy remembers it, being chaotic. Even having perhaps Walker resembling what he remembered Jorgon, being completely upset whenever a rule is broken. Even going as far as to put people in jail, known in fairy world as Abracatraz. While Timmy wished for a new life as Danny Fenton, Cosmo and Wanda may actually still be around, perhaps, they are still watching him. In different forms, such as frostbite, or clockwork, maybe even Jasmine.
As for Danni, she may resemble something Timmy's parents wanted. They wanted to have a little girl, when they got a boy. Some of the Ghosts don't really stray from the fairy's path, such as a wishing ghost. Some ghosts are just ridiculous, such as a box ghost, an unlucky ghost, etc.
But, Vlad doesn't really seem to fit does he? Well, in fact he does. He is what Timmy remembers Crocker as, a demon, taking fairies'/ghosts' powers, and using them for his own personal gain. But Vlad's gaining of the power was quite accidental, just like Crocker's obsession for fairies.
Tucker and Sam were supposed to replace Timmy's old friends, but something went wrong. For Tucker seems to have taken from both Chester and A.J. The food lover, and the tech geek.
The reason they were on some games together, could possibly be because of some sort of time paradox.