New content for Splatoon will be a recurring theme throughout the summer months, with modes, stages and weapons all on the agenda. It seems there'll be some fashionable extra outfits, too.
Magazines in Japan have made some fans in the country happy by confirming that Squid Girl - Shinryaku! Ika Musume - will be included as free extra content in August - there'll be an outfit, of course. It's a popular manga series, in particular, and the Inkling cross-over appeal and familiarities (with tentacles for hair) are fairly obvious.
We're not entirely sure whether this content will come to the West, or whether it'll be branded in the same way, but this could also play into the message that's appeared in the game that refers to the potential presence of "product placement". Quite what this'll entail in the future is unclear, but we wouldn't rule out equivalents to content such as the Mercedes DLC in Mario Kart 8 or Callaway equipment in Mario Golf: World Tour; there have been various examples in Animal Crossing: New Leaf, too.
If Nintendo isn't in negotiations with NERF, we'd be very surprised.