Depends on Switch really. If it keeps selling like fire, they wont pass up a sequel on switch again
Splat2 support will continue intill summer 2019. Splat3 aint coming anytime soon. Itll be SSB omce per gen game.
I can see a spin off though
I agree with that, although I think they might do more then one per generation. I can definitely see a spin off being released but I don't know what aspects are really that popular with Japan so I don't know how they could do a well received one besides maybe a Splatoon - Mario Party type mini game collection.
However I do think they will continue to milk it some how some way going forward. But given half a year later and it still places in the top 5 of Japan's domestic sales, I doubt they are in a major rush.