My suggestion is a new option possibly implemented into the arcade as a new gate. A challenge arena gate which only has arena levels of increasing strengths and varying types to challenge player abilities. The arenas should be classified into different play types ( gunslinger, swordmaster, bomber) and a high score should be kept for players of each type, and the arenas should also have a varying level of space. The more space and less environmental hazards a player has the easier any fight will be. Given enough space and no hazards virtually anything could be beaten easily. So it would be a good idea to have some consistent way to vary both as well as some way to consistently increase difficultly possibly by basing the monsters directly on the chosen loadout for the arena. Where the final levels force the player into a smaller hazardous space versus monsters that are harder for him/her to beat.

I felt it would be a fun addition to the game, I personally think every real time action game needs something like this, so thanks and tell me what you think.