Special treasure rooms: the future should contain special treasure rooms that have it's own item pool with stuff like Steven, lil Steven, time itself, and any other items relating to the future or Steven.

Floor varients: maybe reference to the end is nigh areas.

Availability on different chapters: maybe on each II level there could be a smaller 20-25% chance to spawn the Steven. Depending on the chapter would bring you to different areas in the end is nigh. For example being in mines would bring you the the arid flats. Gheanna would bring you the golgitha. Downpour/flooded caves would bring you to overflow ect.