Lots of games have them, it's the option to play through all the bosses you have defeated in the game. This option would be available to players who have defeated all the bosses (excluding shadow lair, event, and expansion bosses)
Lineup:
D17 Ironwood sentinel
D18 Razwog
D19 Collector
D20 Maulos
Arsenal station floor
D21 Snarbolax
D22 Sputterspark room 1
D23 Royal Jelly
D24 Arkus
Arsenal station floor
D25 Roarmulus twins
D26 Sputterspark room 2
D27 Big Iron
D28 Vanaduke
Arsenal station floor
Between each room would be healing pads, an arsenal station (every 4 floors), standard and sprite alchemy machine, as well as prize boxes (the more bosses you beat the more boxes in the next room).
There would be another arena where all the previous monsters are buffed by the Swarm.
There would be a last one that would have all the monsters of the Swarm run but there would also be event bosses thrown in. First event boss to show up would be Groat/Dread Velvet and the last event boss would be the Apocrean Harvester (here you have to kill it). (You have to have KILLED all these bosses (regular, swarm,and event) before you can purchase this expansion; yep you gotta face that Harvester for real.)
None of the bosses drop tokens or whatever special drop.
The reward for these would be numerous boxes after beating each one (plus a single token from each token boss). The Swarm version would have double the reward boxes. Event/Swarm would have rewards similar to the regular Swarm (just with more rooms).
Both versions would require that you have faced these bosses before. Once you complete the regular boss missions only then is the expansion pack available.
This is aimed at endgame players who want to test their skill and power while remembering the first time they fought those monsters, and want a nice reward.
Note: I'm willing to work with the lineup so long as Ironwood is first and Vana is last.
Edit: Boss lineup and arsenal station distribution
Edit2: Event bosses included with tight requirements
This is aimed at endgame players who want to test their skill and power while remembering the first time they fought those monsters, and want a nice reward.
this won't work until there will be arsenal stations inbetwen bosses, cause you're always going to have the weapons that kill the boss quickly. remove the arsenal stations.
since they are only bosses, we can put more than one in a depth.
swarm rewards should have all red boxes instead of just boxes, just like SLs.
put arkus, maulos and vana apart from each other, since they are all weak to blitz it would make an entire section quite easy, and their patterns are very similar to each other.