25+11 levels take 2-3 hours. It would be great to have some shorter game modes. May be even just single level.
It can go like that:
1. Select difficulty and mutators (special rules, like "Angel of " something)
2. Select stage - one of special stages, or random stage with level, type and subtype (ex. level 15 cacodemon cave).
3. Get gear - one of presets, or room with random drops, or point buy.
4. Choose class and traits, depending on stage level.
And then jump into action.
Setup can be exported to a string (with or without random seed for level and gear, if applicable) and shared with other players as a "challenge" or "challenge pack" (i.e. several presets to play consequentially). Game can have some challenge (packs) "pre-installed", with rewards for completing them in form of badges and new stages, mutators etc. unlocked.