During Microsoft's E3 press conference, Microsoft announced Cuphead DLC along with a new trailer. It will be coming Spring 2019 and I'll update this post with more.

[UPDATED]

That (obviously) didn't take long. The new trailer is posted online and we got some details straight from Studio MDHR, including that the game will have new weapons, charms, and bosses. The game will also feature a new playable character Ms Chalice.