Edmund McMillen has been busy. Six weeks ago, he released the final free content booster pack for The Binding of Isaac: Afterbirth+. A few weeks prior, he and James Id released the first trailer for their upcoming match-four dungeon-crawler, The Legend of Bum-bo. And a few weeks before that, McMillen said his delayed, cancelled and now revived game Mewgenics is back on track. Apparently McMillen's still got free time, though, because judging from a recent string of tweets, he's got a new multiplayer game in the works.
6 more days... (till super secret game explosion of amazing!)June 14, 2018
a few fun facts: this is a new unannounced game, its multiplayer, fresh but familiar.June 14, 2018
With its official announcement still a few days away, little is known about McMillen's mystery project, but he did share a few interesting details on Twitter. Firstly, it's not connected to The Legend of Bum-bo, The Binding of Isaac or Super Meat Boy. And for those wondering: no, it's not a battle royale game. McMillen also held a dedicated Q&A, and assuming his answers are only half-joking, his mystery game is a turn-based, local multiplayer PvP game that's not a platformer or a shooter but is based on one of his old games. That oughta narrow it down.