We've just announced our plans for replayable Legendary events, and we call them "Mythic Events". Please use this thread to discuss and leave feedback for the first one: Pieces and Plans Mythic Event....
That logic doesn’t make since to me, beings mods have been in game for a long time now. If that is intentional then they need to eventually rework Mod Challenges to be a more effective way to farm decent/good mods.