I don't like the passive-aggressive tone of this thread. OP, if you don't agree with another poster, just say it and discuss. If you genuinely want to compare Splatoon to other multiplayer-focused Nintendo series, you should use another tone. There's also a huge difference between "with this instalment, they're playing it safe. Still, those minor additions look cool" and "it is a garbage series".
That said, since Double Dash!! every instalment of (mainline) Mario Kart has significantly improved the series.
- Double Dash: Overhauling of how the player controls the kart, better item balance, non-standard karts. Even without the two-character gimmick (which added nothing to the game, as cool as it was), this was a major gameplay reboot;
- DS: Built upon DD with online mode;
- Wii: Tricks. This was a major introduction -- before, the surrounding environment was mostly a hazard; now, the player is led to interact with it;
- 7: Tricks were both de-emphasizing (the player is no longer graded) and integrated into the level design, sometimes allowing you to glide. By gliding, the player can now skip part of the tracks -- effectively creating a parallel, "aerial" track. Water sections on the other hand were a non-consequential gimmick. I concede 7 was a sort of "bridge episode" between Wii and 8;
- 8: Anti-grav was mostly inconsequential. Mostly, the game was a more focused, better-realized version of 7. Also first HD instalment.
For example, I re-played DS some months ago and I really noticed the lack of tricks. Their introduction really changed how the player interacts with the environment. 7 and 8 built upon this introduction.
Smash Bros. took another approach, with each episode having a different, ambitious hook. Plus, of course, more and more fan-favourite characters:
- Brawl: Subspace Emissary;
- Wii U / 3DS: One game, two versions;
- Ultimate: Everyone is Here!
Splatoon 3 seems closer to the Mario Kart approach, but at the same time what they show seems minor additions to the formula, rather than major improvements -- something you would expect from a QoL update.
Still, I think that neither the MK nor the Smash formula is what Splatoon should do with its sequels; rather, I'd like the series to give a "Splatoon twist" to existing F/TPS genres or concepts (e.g., a battle royale "SplatNite" mode) or subverting its rules (e.g. a limited-ink mode, or a "Weresquid" mode where you can't turn into a kid at night). And of course, expand the single-player portion, which is part of the series' identity (to be fair, today Direct has shown almost nothing, so it's hard to tell what they're doing in this regard).
