My biggest takeaway from reading this whole thread and thinking about this deeply:
The "filler formula" from the first four generations has truly started to dissipate in the last three generations, making it harder to classify what's filler.
In BW, XY, and SM, episodes that initially may seem to be filler based on our earlier definitions from the first four generations are truly not so:
Stephan initially seemed to be a COTD and yet became one of Ash's main rivals in BW, so much so that he gave him his best battle of the league. There are many other BW examples as well.
Bonnie's episodes taking care of Espurr, Flabébé, Lapras, and Tyrantrum were building up to her taking care of Squishy.
In SM the examples are too numerous to list out all of them, largely assisted by the "home base" formula of most episodes taking place on Melemele Island, but very frequently earlier episodes ended up tying in to more important episodes later on, that you wouldn't have noticed at first if you just take the plot at face value, like Mallow's first Oranguru episode introducing an important Pokémon who's not just a POTD, but appears many times later, including ending up giving Mallow her Z ring.
too many at a time are thrown at us like the ridiculous amount of time it took for Ash to battle Candace in DP due to enormous amounts of fillers.
Between Byron and Candice there were only 17 episodes, only 7 of which were filler.
Yeah I dunno why the writers couldn't handle developing Team Magma and Aqua back then. They had so much game material that they could've adapted and it all went to waste.
It's because it was the first time they were trying to write a "serious" villain arc. In Kanto and Johto, outside of the mini-arcs at Whirl islands and Lake of Rage, there was no overarching "arc" for Team Rocket, and the TRio were treated as comedic villains. Being told by higher-ups to handle multiple things at once, none of which they were used to, probably made it challenging: writing an overarching "serious" villain arc for once, handling multiple teams (three), fleshing out characters who were meant to be true villains, keeping TR in the picture even though they're not in the games. As you can see, there were multiple challenges they dealt with that I just listed out. As time went on, they got better as we can see with Team Galactic and Team Flare, and I'm also convinced Team Plasma would have been much different and better if it weren't for the earthquake cancelation. It just took experience. That's the simple answer to your question. It was the first time they were trying such an arc and that's why they messed it up.
in XYZ it lacked even the over the top comedy and self awareness to keep it going a little.
Really not sure what you're talking about here. I find XY29, XY37, XY71, XY79, XY84, XY&Z21, and XY&Z22 to be some of the funniest fillers in the show, most of them largely because of how over-the-top they were. I find myself rewatching these more than I rewatch a lot of the key plot episodes from XY&Z.
Let us make a defenition for Pokémon fillers ok? What is a good defenition?
A good and easy, hard to dispute definition may simply be: an episode that you can skip, and you'll never be confused watching any future episode.
However, as
@Pokegirl Fan~ notes below, discussing what is an isn't a filler isn't the only problem.
Its why we should label these 'formulaic cotd/potd/Team Rocket episodes' over filler/non filler instead.