Ban customs WITH windboxes. Of course moves that already have windboxes wouldn't need to be banned; that would be ridiculous. But as custom specials with windboxes are optional and can be finnicky, banning those shouldn't be a problem nor a hindrance (Except for Dr. Mario using Soaring Tornado)
Fast fire bird, explosive lasers, and void reflector are really good tools for falco that opens up a lot of stuff for him, he also has the option of using an invincible phantasm in edgeguard heavy mu's. I wouldn't call him a loser in the customs meta
Besides DLC characters, the only one that really gains nothing is Metaknight. Not a single viable custom except for High speed drill, but it's a dubious side grade to a move that never gets used in serious play.