I fully expect a substantial DLC pack for it some time in the next 6-12 months, rather than a standalone game. I'm sure there is at least one interview where Nintendo were pretty clear that - had the mechanisms been in place - they would have made SMG2 a DLC pack, rather than a whole new game. Now they are in place, and now that Splatoon is clearly a success for the company, I expect that some kind of expansion pack will hit before a sequel does.
That does a few things:
-- It keeps the playerbase engaged, draws in lapsed players and adds more content for anyone on the fence.
-- It allows experimentation within the confines of the existing game framework without the pressure and expectation that comes with a whole new game, allowing them to test ideas and modes.
-- It lets Nintendo re-use existing assets and the engine, plus whatever tools they have specifically for this title, so they can have a (relative) skeleton crew on the DLC rather than a large staff.
I'm also expecting some kind of limited "gift a friend" offer at some point, with Nintendo pushing to expand the playerbase by either offering a download code to existing owners, or by offering it alongside new purchases. I'm pretty sure they did that with Monster Hunter in the West, and I think it's a worthwhile tactic when it comes to drawing people in.