It's a cape obviously.

These could be given out to players who have defeated the expansion mission. This mission could be laid out like the Gauntlet.

Prior to the mission you could choose a monster family or a status.

The monsters in the mission would be spawned based on which you picked.

Example 1: You choose fiend. All kinds of fiend monsters spawn of all status types.
Example 2: You choose poison. All the monsters that spawn can deal poison damage.

Note: Silkwings and Menders spawn in their regular spots.

All the capes you earn have the emblem of whatever you defeated (poison icon on toxic themed cape for beating poison arena)
Note: Curse is not included as a status choice, rather you receive this by collecting all the capes.
All the capes you acquire are bound to you much like the various crests you acquire through story missions.

In addition to this you acquire an Elemental/shock Magnus gun (2/3/5 star versions), and a Pierce/poison/freeze resistant bomber suit (comes with fire/shock debuff) (with CTR med and Damage low(2/3/5 star versions). (no new mechanics just another little prize from the mission).