The big promise of Back 4 Blood over previous co-op shooters like Left 4 Dead is the game's live-service support and layers of progression. Left 4 Dead players already play the same content week to week, so it makes sense to reward them for that and roll out fresh experiences to keep them coming back for more. Back 4 Blood, however, didn't cover all of the ways that Left 4 Dead players like to play. Namely, solo players are locked out of these progression systems. That's going to be changing in December, however.