How about an arena in Manderville Gold Saucer, but using a 16-bit battle system.

instead of one or two battle, how about a sequence of battles like in Gold Saucer in FFVII? And do things solo or with a Light Party (1 tanker, 1 healer and 2 dps), but if solo put less difficult

But how some skills work in these battles and how this battle work online?
This battle should work in an Active Battle System (that option even you still choosing a skill, the battle still running)

Well, I dont know if some skills should work like in old FF games like Fire 3 hit everyone, but no use of Astral or Umbral Ice and monsters have elements
And how tanker will protect party? some skills like Shield Oath can have the effect of attract monsters to attack him/her. (just an idea)

How about rewards? Every battle can give some mgp and can work like this, for example:
"You are in 10th battle, if you stop now you will receive 500 mgp, but if you fail in 11th battle you will receive 250 mgp".