I've said one numerous occasions, in a perfect world, when a player sits down at their computer for the evening to play the Diablo the question I want them to say to themselves is "What do I feel like doing tonight?" not "Ok time to run Act 3 again".
It's not exactly a perfect world, though. It's doable. A good place to start is a map system. Map systems are great, it doesn't matter who or when invented it, it's a pretty solid system and you shouldn't be ashamed of adding it because certain other titles make use of this mechanic.
Allow maps to drop at a fairly decent rate from any Act. Any mob. Like any other item. Which will function well with the increased mob density in Act 1, 2 and 4 in the upcoming patch.
These maps create a randomly generated dungeon in which the player has increased magic find, a large number of elite and champion packs can be found inside these maps and make sure the map has a decent size, so each map takes about 10-15 minutes on average.
Add legendary maps that take maybe 30-45 minutes and throw a boss or two into them that has a guaranteed legendary drop or so.
Allow these maps to have affix for increased density, difficulty increases and so forth which in return gives you a higher magic find and perhaps even bonus drop % on the map. For example a legendary map with 30% bonus drop % plus MP10 will allow a single mob to drop up to three items. Separate roll from the default MP bonus drop.
Hell, even add crafted maps, allow maps to be rerolled by combining several maps, let players uptier them, make people farm certain acts for certain maps, something. Give us something! :p