Maybe this has been said before, I'm not sure, but adding in-game, meaningful leaderboards to end-game activities would be great for re-playability. Ex. For each tier of the Nightfall (including GM), have a display of a global player list ranked from certain metrics like time, score, etc. Also include the build that player last used to complete that activity and obtain that score/time. Stat trackers for each activity that display your global rank would also be pretty cool! In addition, for players who reach high ranks, give them extra rewards beyond regular rewards. Perhaps unique emblems or skins or additional shards?