Not super different from Spla2n based on the battles and salmon run matches I've played so far. At this point it's beating a dead horse but I don't think this game is different enough to justify a new entry. All the changes i've seen between the new specials and salmon run bosses could have easily been implemented into the already very successful prior entry.
It's still fun cuz it's sploon but it just seems like taking advantage of a loyal fanbase when this game has no real hook to differentiate from it the other two entries.