Maps and the hideout
After the campaign, the game is maps, opened from your hideout, where friends drop in to join you.
Art in progressAfter the campaign, the game is maps. You open them from your hideout, and friends who come into your hideout can join the maps you open.
Today's game already has the core of it: every depth is a new cave generated from a seed, a portal takes the whole party further down, and depth has no floor. Monster life grows 12% per depth and compounds: a Cavern Brute with 60 life at depth 1 has 1,605 at depth 30. How deep your party can go is the measure of your builds.
Proposal
Maps are items. Each one opens a single generated cave with a tier and modifiers, and its tier is its depth, so the scaling above carries straight into the endgame. Maps roll and craft with the same currency as gear: more monsters, more dangerous ones, or more loot, traded against each other.
The hideout holds a map device, your stash and your trophies. Opening a map puts six portals in the hideout; anyone there can step through, and a fallen player comes back through one of them. When the portals run out or the map is cleared, it closes.
Each season, the deepest map tier a party clears goes on a board for your friends to beat.
The maps also go down into the Unlit, under the floor where the fallen wake, which has no bottom.