I think there's a difference between 'good filler' and scenes necessary to preserve continuity and follow the rules of good cinematography.
Here's a hypothetical example:
Scene 1 - Luffy talking
Scene 2 - Shocked reaction face (filler)
Scene 3 - Luffy talking again, but from a slightly different angle.
Your first attempt might be to cut scene 2, but the result would be very jarring. Transitioning from one scene to an almost identical scene is a big no no. Even worse if a background character or element moves during the gap.
You could shorten the reaction shot or even keep it entirely if it's not overly long. Sometimes this is the only option depending on timing and music.
My favorite is a technique I call "having your filler and eating it too". Replace all or part of scene 1 or 3's video with the video from scene 2, while keeping the sound from scenes 1 and 3. The result would be something like:
Scene 1 - Luffy talking
Scene 2 - Luffy continuing to talk while a character silently/quietly reacts
You can use whatever you want instead of reaction shots. Maybe unused establishing shots, characters from behind, pull in clips from entirely different episodes even. Even though you're "replacing canon scenes with filler", doing so can make edits seem less jarring.
