So I noticed that in the reaper mode that mysterious remnants seemed to be dropping like candy.

Instead of having the mobs drop mysterious remnants have them drop "reaper remnants" instead, or have the quest reward them on completion like the challenges do based on some metric.

Then place a "Reaper Trader," just like the mysterious remnant trader, into the Hall of Heroes.

Have unique cosmetic rewards that can only be obtained this way.

Things that looked really evil and reaper like. Like awesome skull helms, death wraith outfits and the like.