An Epic Games job ad puts out the call for a Senior Systems Designer for one of two projects, one of which is Fortnite, but the other is an unannounced  "competitive online action game that includes player progression, heavy itemization, and a dynamic economy" (thanks VG247 via NeoGAF). That's about all it says about the game for now, but here's the whole shebang:

Epic Games is looking for a Systems Designers to join one of two projects. One of the teams is working on an unannounced competitive online action game that includes player progression, heavy itemization, and a dynamic economy; the other is for Fortnite. In either of these roles you’ll be working on the biggest most important systems and will have a hand in designing everything from economics and itemization to balancing combat mechanics. You’ll be responsible for general design, balance, rules, interface, etc. and will play an integral role in defining the outcome of the project.