Nah, having an arena style event tied to the Golden Saucer would make more sense in this case than having yet another currency. Lets play "what if" for a moment.

What if the arena rewarded items that were BiS or at least on par with current content, then people would feel the arena would be mandatory and SE has taken a stance that everything outside of the Main Scenario quests are not mandatory.

We have enough ways to get high level gear so adding another set of gear would either be specific to the arena (like the PVP gear for Wolfs Den) or purely for glamour. Since we already have glamour weapons for sale with MGP, what would the point of adding yet another currency be? We already have Gil (buy crafted gear from other players), Tomestones (weathered/ironworks), Raid drops (Coil and CT), Wolf Marks (for PVP), and now we have MGP for the monster weapons. Do we really need another currency for yet another set of glamours? I am going to vote no.

There are a fair number of people begging for other ways to earn MGP, or changes to how MGP is earned currently. This would be a way to address several requests at once. Increase the way MGP can be earned, add arena style battle content, add more weapon/gear options for glamours (perhaps even outfits from previous/future FF games).