I think a lot about the "every game needs a new gimmick" thing and specifically what makes it different from the general idea that sequels should, like, iterate on previous work
Like, a ton of shit changes between the first two Ratchet and Clank games, right? But they still feel, imo, largely cohesive as two games in the same series that are in conversation with each other. And by and large I think this holds for the series. Even when the Future series started, you can see the DNA of the first game in there. There's different guns, different mechanics, they try new things that don't always work and throw them out the next time, but they feel like a process of trying to make the best R&C game possible.
I don't think this holds true in the same way for, say, Mario or Zelda or Pokemon.
That's not to say they never iterate, obviously, otherwise we would still have Mario 64's unusable camera and Ocarina of Time's basic combat and Pokemon's uh.............
Anyway
I guess it's like despite them being released back to back, I feel less lineage between Mario 64 and Sunshine than I do between any two Ratchet and Clank games
Or Sunshine and Galaxy
We spent all this time and effort on the FLUDD mechanics and now we'll never use them again not because they didn't work but because we're doing this other thing that we will never use again after
Okay I joked about Pokemon but honestly that's a great example of it in action
Every new gen now there's the new big dumb gimmick, there's Z Moves or Mega Evolution or whatever else and they all do slightly different things and you'll never see any of them ever again
Lady, EVOLVED 
@coriander pokémon does it for slightly different reasons tho, or at least to me it feels more justified, because they really just want every region to feel unique. each region has its own food (and related food mechanics) and, since Gen V for serious, its own way of pokémon battling (rotation/triple battles, mega evolution, Z-moves, Gigantamax, Terrastal).
there are obviously commercial reasons why they do this; it feeds into the marketing and gives them new things to show in the anime and inspires new TCG mechanics (which powers card sales) and so on. but “i want each region to feel unique” is at least a reasonable artistic motivation to have even if the execution is sometimes lacking.
that’s different, in my mind, from mario, which is like “there is a powerup which lets you become an elephant this time”.
