In addition to boxing gloves, I suggest a specific item that would give a reason why players would play this minigame. For example Arcade has Link set, kunckles pet. Battlegrounds has Capt America shield Thor's hammer, rogue set etc.
For my suggestion it would be
1x Pet for each side giving a stat, example lets say this pet is able to perhaps fit the role of the collectors necklace and pick up items for you and store to your bank. Or perhaps it can involve something to do with the double drop or drop rate.
My pet in mind would be blue side : Stitch, For red would be Lilo.
For the set of Armour maybe the black panther suit where the special effect could deflect damage (maybe aoe to benefit arcade). I know that set might not sound as good but with decent stats and perhaps giving it drop rate or double drop would make it a really good set to have.
For a weapon perhaps thanos's infinity gauntlet where each attack could be different or there are different attack modes (something like how the kbd has different attacks)
Many like myself love the cosmetics in this game and would love to see something like this added. Many of us are marvel fans and this would attract lots of attention to castle wars.