I personally don't care for crossovers, but I've always pondered on a Danny Phantom/Teenage Robot special, because the angle of two Nickelodeon superheroes meeting via time travel (Clockwork fits in here)--whether sending Jenny to the past (present time) or Danny to the future--could, if written well, be something entertaining. I'd mention El Tigre (another superhero show on Nick), but I have yet to watch it, so I can't say on his behalf.
I actually don't see a Danny Phantom/Fairly Oddparents crossover. They may be from the same creator, but I feel the two show to be so dramatically different from one another that I just can't see them compatible.