Rock Band 4 has been down this road before. Like Fallout 4 before it, Rock Band 4 is crossing over with Gearbox's upcoming MOBA-FPS hybrid, in the form of the Battleborn characters, Thorn and Miko, who will be available as playable characters in Rock Band 4 as DLC. The announcement was made by Gearbox's Rock Star CEO, Randy Pitchford at a panel during PAX East today.
Pitchford didn't share much more than that, but he did say that there will be a Harmonix panel tomorrow, Sunday April 24th, where the Developers themselves will go more into depth. Stay tuned for more information on that.
PAX East has been a very busy weekend (and as such Twitch planned accordingly) with confirmation of Borderlands 3's development as well as the possibilities for The Banner Saga 3. More announcements are sure to follow as the weekend rolls along.