Posted this back it March but it never saw any traffic.
I had the idea to make an arena that can accommodate for any boss in the game.
This arena would be accessed by spending 150 eridium to spawn ANY random boss as is to the arena to fight. No guarantee on who or what you will get. This boss is in no way different than the one in their respective areas. Once defeated you can go back and re-spend to get another random boss.
Then a player can spend 300 eridium for an invincible variant on a boss. This can be any boss or an existing invincible boss. The loot pool will be increased with the difficulty spike for invincible. Once defeated you can go back and re-spend to fight more.
Once a successful run is done the path back to the starting area is has eridium laying about it (about 200-300).
On a successful Invincible run an added side room containing an extra 300 eridium and about 5 red chest will open up.
A player could spend 1000 eridium for a continuous boss rush in the arena. Bosses will spawn until player defeat or all current bosses (outside of invincibles) are defeated. Once complete players can return to check loot or start up again.
Finally a mode for continuous invincible boss rush costing 1500 eridium. Bosses will spawn until player defeat or 10 bosses (with all being an invincible version) are defeated. Again this will be repeatable.
After a successful completion of either boss rush mode players will be rewarded a loot room containing eridium (2000+) and 20 or so Red chests from each area (DLC) within the game. A run on the invincible will give a bonus 1000 eridium and 10 chest that are like Lilith’s chest ( guaranteed legendary).
Invincible bosses will function similar to their regular counterparts but will do increased damage, have more health, and do special attacks more often. They will also contain a legendary or unique drop only attainable with the invincible tag on them. (Might be a good way to add back Pearlescents)
Obviously this will need to be refined and work shopped to get down.
This is an idea for a way to run through all bosses or a challenge to see if your gear setup can handle anything and everything the game has to throw at you.