PvE Stadium could be good as well

General Discussion
1
  1. It just feels like Diablo PvP.
  2. As with other build-oriented ARPGs, most people in stadium also care only about their own builds, not others. It’s too complex to calculate/predict things, and thus there’s less meaningful PvP interaction than the regular game imo.
  3. When the gameplay is focused on the builds, then it might be more fun to play hack&slash against AI mobs.
  4. Balance will often be the second citizen in such a game mode and this would conflict with PvP so hard. Various choices will be meaningless because only the most effective and the best possible builds will be used, otherwise you’re likely to lose. But it’s less of a problem in PvE.

Not story mode. Just like the greater rift in Diablo 3 without difficulty level, and the mobs will be stronger as you progress.