When I first saw the update for cultist origin story I got an idea of new DLC that would be focused on Davkul:
Idea is to create Davkul end game crisis that would be fulfilled by enough sacrifices of brothers in the game, there would be inherent number (X) of sacrifices which would be slowly progressing the event of portal opening (it could be specific quest location for final sacrifice).
Potential way how to get to the opening:
Player is not playing cultist background – there will be another party of cultists generated that would be roaming the land and slowly generating sacrifices. If the player party kills the party there will be another one generated in time X to continue the purpose of sacrifices.
Player is playing the cultist background – Player would be managing (random) the sacrifices, getting boons and finally progress to the final sacrifice to open the portal.
The final sacrifice could be event that would possess the whole party giving in-human stats (X% increase in stats on your decision how to balance) all of the cities will become hostile and your company will be fed through flesh, no recruitment, you need to finish with what you have, and your mission is to burn the world.
If you don’t play for cultist your mission would be to kill the party that spawned the portal and close it. In addition the portal would be spawning parties of (depends on how you want to theme this) either possessed units that are already in the game (mix of orcs, humans and goblins together that are driven mad by the call of davkul) or directly lesser demons that would pour out of it, roaming the land trying to kill whatever they see.
Portal would be closed by legendary fight on the location with either phased davkul that would be trying to get through the portal and there would be some battleground elements that would be needed to activate to close it, while you would fight possessed enemies. If not Davkul himself, the party could be fighting some higher demon of your choice that would be trying to get him in and end the world.
New weapons: focused around the cult, sadistic theme (scythes, new whips or iron staffs)
Hope I haven’t run too wild with it to be within the design possibilities.