Players have already shown a desire and ability to arrange one-on-one matches, so why not set up this option.

Call it “Gladiator” mode perhaps. It would be a big circle or octagon.

Here are somoe possible options, but not requirements:

Perhaps it should cost energy, say 100 energy, to play a match. The winner gets back 150 and the loser gets nothing.

There should definitely be a challenge mode, as well as an all-comers mode.

Melee matches would also be very, very cool.

You could even add monsters generator pads to interfere or add another kink to the game in order to require a bit higher level of fighting skill, but only the slow monsters like zombies, constructs, jellies, slime, and basilisks.

Finally, gear should be PvE gear, shields, UVs and all, with shield bumps et al.