Idea Factory International just confirmed the development of a new Neptunia spin-off on their Twitter page. It's called "Hyperkart Neptunia" and is IF's first racing game. The game seems to have a new artstyle as well and will be released in 2016. They've teased a picture of the game on their Twitter page as well.
Are you as excited as I am? We haven't seen a new Neptunia game for an entire month, so it was about time.
Source: https://www.twitter.com/IdeaFactoryIntl/status/583337164211011584