Not gonna lie, while the gameplay sounds interesting, I'm just not too into the tentacle creatures, and I hope they focus more on Ink/Octolink like humanoids in the future. (minus the shopkeepers I guess, but still). Real world only has one sapient species, let alone dominant. How many can they add before it's over the shark?
In regards to S3, I honestly to wait a bit on this one. Remember the same staff does Animal Crossing. I think the only reason we got S2 so soon was because the Switch launched so oson after the original's release and they wanted their shiny new franchise on it ASAP. Now that Nintento's in a stronger position, I'm confident we won't see another mainline game until the next gen.
As for spinoffs- yeah, Inkling's in Smash. But so are Captain Falcon and the Ice Climbers, that really doesn't say much. I'd put more stock into them showing up in Mariokart to be honest. Only other franchises there are Zelda and Animal Crossing, that's much bigger name company. Even most of them only see one or two games per console though. Zelda, Nintnedo's most consistently succsessful and supported franchise after Mario had: Ocarina of Time and Majora's Mask on N64, Twilight Princess on Gamecube, Skyward Sword on Wii, and Breath of the Wild on Wii U, because that's what it was for originally. The last time Zelda had two games on a system was literally in the 90's. And Splatoon has an issue Zelda and the others don't in that regard, a reliance on other players. I only bought Splatoon 2 a year ago or so, I missed a significant portion of it's supported run, and that was with the extra incentive of being on a new system.
If Splatoon 3 is released on the Switch, it will not be an immediate transition. For a timespan of years, probably all the way until they shut the servers down, the existence of two games on the same platform will just split the player base. Leaving the lobbies and communities for both smaller. Splatoon lives and dies by it's multiplayer, a reduced network affect is a serious problem. The existence of S2 would prevent S3 from reaching it's full potential as long as people still wait to transition, and S2 would be seriously kneecapped by S3 existing on the same hardware.
While I'm writing this though, here's what I want from the game most of all: Ranked Turf War, and a casual version of the Ranked modes. I shouldn't be forced to play Ranked just because I want to play Splat Zones or Tower Control, and I know alot of competitive players don't play Turf War as much because of the casual players there. This would solve that.