Isn't this exactly what happens in matchup threads? The old idea was more unique since we were generally discussing stage advantages themselves. This one is definitely a matchup thread.
Not really. Knowing where to take characters (and why) is rarely discussed in depth in character matchup threads. Usually you are met with "go brinstar..." and that's it in the character matchup threads.
Saying "lucario does well on FD" is meaningless against ICs who WANT FD. Generalizing how well Lucario does on each stage does not help the player when it comes time to counterpick intelligently.
On that note, I guess I will say something constructive and say to consider Frigate and Norfair against Diddy. It is a pretty good Lucario stage (his recovery doesn't get owned here and Diddy's can be), and the rising/falling platform can mess with nanners.
Norfair eats his nanners for breakfast and if you are good at the stage you can easily make a Diddy feel helpless.
If you've experienced a Diddy on Luigi's and feel confident about not getting trapped in his mansion tricks, you can actually outdo him here by storing your aura and pulling ahead on the basis of not being able to be killed.
Japes isn't a bad idea either for the same reasons.
Ban FD, Rainbow Cruise, Corneria, or GG if you see him ever taking people there. Mainly FD and Cruise to ban.
Oh and as far as the neutral stage to shoot for, Yoshi's then Kanto. Avoid FD and Smashville.