Doing outdated content is not going to be a thing. Blizz has said multiple times they don't like people having to do outdated content. However, that we get new quests/scenarios where we might have to go to Outland, Pandaria, Draenor etc, where ever the race is from, I think is likely. However, I doubt we will have to farm severely outdated reputations. What gameplay would it be to go to Outland as a lvl 120 and kill lvl 70 mobs? That being said, having to grind a current content rep is not off the table.