Since leagues are all the rage atm: How about running "leagues" for single player as well? It'd probably work best as preconstructed events, running for a longer time, comparing the best runs (fastest time or best vp scores?) of the Challenge. You have a certain amount ot attempt to replay a challenge to better your score - say 4, to keep it in line with the current League system, no tiebreakers. The fun bit is that it doesn't have to be balanced and symmetrical in terms of power, so you can pretty much run this as hardcore adventure (of varying degrees). You can also apply several of the Quest parameters for these runs - if possibly giving extra score if you manage "no deaths" et c, or have them work the same as in Quest Mode. If you select No Deaths, you can get a higher score - but you use up one of your attempts and have to restart if you fail to keep everyone alive. Entry fees and prices are all up in the air, depending on what the Leagues wind up at - but I could see a guaranteed epic for first place, guaranteed rare for second - in classic adventure styled loot chests for instance. The rest would get a standard loot chests, if they manage to beat the adventure that is.
It's been poked at before, but I think the simplest implementation for single player would be to run some of these variants as weekly challenges. I would totally pay pizza to "subscribe" to a rotating set of single player challenges that award an Epic chest.
