The GCN games to be fair did add some things to the table like Shadow Pokemon and being more mature than other games as well as having a wild west motif. I personally would love Shadow Pokemon to return but eh.Why am I getting this feeling the only reason Pokémon fans here are saying they're tired of the same formula is because of how much they praise the GCN games, despite being "meh" for everyone else because of how it lacks key elements that made Pokémon fun in the first place, and wish the mainline games were more like it?
Though what do spinoffs have to do with Pokemon games basically reusing the same general structure over and over? You don't even need to play any spinoffs to see how things work. In Gen I it was revolutionary. Go catch Pokemon! Fill a Pokedex! Get Gym badges! Beat an evil team! Become the champion! Gen II did the exact same thing, with the added bonus of getting 8 more badges and exploring Kanto again so it was just fine. Gen III... same exact thing as Gen I except in Emerald you fought two evil teams instead of one and got access to the Battle Frontier, and in Firered asnd Leafgreen you went to the Sevii Islands and wrecked Team Rocket and found precious stones. Gen IV was also like Gen III except you got to go to the Torn World and got access a Battle Frontier that was connected to a bigger island where you could beat up Team Galactic again. Gen V was like Gen I, except we got PWT and the story was actually very good and important and we got new places to explore, and Gen VI was just a half-baked version of Gen I and III with no Battle Frontier and with a convoluted timeline plot involving Mega Evolution. The only thing special about it was the lore.
Do you see a pattern here? All of the main games followed the exact same structure: fill a Pokedex, get badges, beat an evil team, save the world, become Champion. Maybe the places you can go are different, maybe the evil team is different, maybe the Pokemon you can catch are different, but in the end you're doing the same things over and over again. The only generations that shook up this formula were Gen II for adding a second quest and Gen V for having the main story front and center. Is it any wonder why people are getting bored? Playing through a Pokemon game the first time is an amazing experience, but after so many repeats of the same thing it gets a bit stale. Gen VI didn't help things by having an underdeveloped region, a poor man's version of Team Galactic, a tacked on aesop about resorce scarcity that didn't belong, too much handholding, and shoehorning in a feature without fully explaining what it is or exploring the implications of this feature (Mega Evolution, if anyone's curious.)