yes i think there'll defs be a splatoon 2 somewhere down the track, and I kind of expect it to be revealed at this years E3, if not then E3 2017 at the latest. It was a major success and it wouldn't make sense for Nintendo not to keep milking it by making a sequel.
That said there are a lot of things that I think need to happen in order for the game to move forward. They need to cut the DLC BS. I really loved splatoon, but I stopped playing after a month because I got bored of waiting for new content. Not to mention, it was even more annoying knowing that the majority of the content they were pushing out for months after release could have been included on the disc day 1, but they didn't because they wanted to be annoying. I can let it slide since this is the first game, but if this happens in the next, I'll be mad.
There also needs to be more of a single player mode. Again, I get this was their first game, so I can let it slide, but the next one really needs to have a much better single player mode - I'm really not a fan of games that rely heavily on multiplayer.
I also think a lot of the gamepaly (more so multiplayer) needs to be changed up, too. I'd love for turf wars to be a lot more "personalized", and be able to last longer than a mere 2 minutes, and I'd also like a lot of the elements from single player mode - like sponges and zip lines - to be included in multiplayer courses. That way they'd be a lot more strategic, and not just a "go out and shoot everywhere" feeling - I know there are some courses like this, but like I said, I stopped playing after a month so I never got the chance to play these maps - and speaking of maps, get rid of the stupid "2 maps every 4 hours" crap. It's annoying and pointless and shouldn't still be in the game 6+ months after release.
More weapons/gear/maps/gameplay styles are obviously going to be in the next game so I really don't need to go on about those, but I feel like product placement could work really well in this game. I know there's a little already, but this game really needs a mario sunshine cross over (wait for sunshine 2 announcement and it will happen !!), but I'd also love to see games like Zelda, Animal Crossing, Metroid, Donkey Kong etc... be thrown into this game. It'd make it a lot more diverse.
Something that I also could've sworn from what Splatoon was first revealed, was that it was going to be on the 3DS and Wii U? But I guess I was high or something because that's very clearly not the case. But I really hope the next game gets both a console and hand held release, the 3DS is still going strong, and I can see Splatoon work really well on the 3DS (especially now since we have the new 3DS with the second circle pad, controls will be easier), so I really hope whenever it's revealed it gets a cross platform release.