According to a South African retailer, we may see another series of Amiibo launch in May.
The retailer, Raru, currently has listing for "Amiibo - Splatoon Boy," "Amiibo - Splatoon Girl," and "Splatoon plus Amiibo Squid"; this suggests that there will be three Splatoon Amiibo (a male and female character, and a squid Amiibo). It also seems that the squid Amiibo will be exclusive to a bundle with the game, similar to the Mario Party Mario Amiibo; the retailer also has listings for the Greninja and Jigglypuff Smash Bros. Amiibo for a May release.
So basically, there will supposedly be a version of Splatoon that comes with a Squid Amiibo, while both Girl & Boy Inklings will each have Amiibos that are sold separately. On top of that, the Super Smash Bros. line of Amiibos will apparently gain Greninja & Jigglypuff in their lineup, rounding out the default Pokémon crew (of course, there's Mewtwo, but I'm not sure if DLC characters are getting Amiibos).
Source: Gamnesia
EDIT: An error in the quote from the site, the Mario Amiibo that's with Mario Part 10 is also sold separately. Thanks Shauni for pointing that out.
EDIT 2: Raru has removed the Amiibos in question from their site. Take that for what you will.
EDIT 3: Though it's missing the Splatoon game w/ Squid Amiibo bundle, here's an image of the other separate Amiibos that were listed before being taken down (courtesy of Luis_lara12345 of Reddit)
EDIT 4: twothunder got this screenshot through the site's cache.