A couple days ago, when browsing my unread posts, someone's thread title immediately gave me a cool idea for the game. (I forget who, and i didn't even read their thread :p it was something like EtG meets Skyrim; the title had the word 'scrolls' in it anyway, and that's what suddenly triggered my idea)
Anyway, the idea is that there would be ways to win 'items'. These would be one-use things like scrolls, potions, etc, that you commonly see in RPGs. An example might be something like Potion of Vitality: Give +20 HP max to the next (non-PVP) duel you enter. (like how a pet works) You could have things like extra hps, extra quanta to start, an extra mark (2x or 3x or whatever) and so on, things that would improve your chances of winning the next duel.
How would you get these 'items'? Well, I was thinking that each FG could have one item associated with it, and if you beat that FG, there would be a small chance, maybe 1 in 20, of winning an item based on that FG's style. Beating Neptune might give you an item that grants you a Trident in play at the beginning of your next duel. Beating Elidnis might get you an Ulitharid pet. Beating Miracle might give you an item granting +20 hp to your next duel. Beating Rainbow might give you an item that doubles your mark for the next duel. And so on...
Maybe you could get them from the oracle as well, instead of pets some of the time. The idea would be you could save these items up and then use them against FGs, Arena, etc, just like you can pick FG/plat opponents to use your pets on now. It might be a good idea to cap the number of 'items' you could carry around with you, but the idea is that even if you had a near-perfect FG win rate, you still wouldn't get the items that often; they'd be something fairly rare. You could also theoretically use multiple items for your next duel, like if you were at 2/3 rings on platinum arena, you could use a whole bunch of items to increase your chances of getting that third win.
For those old enough to remember Super Mario Bros 3, I'm thinking exactly like how you could get this inventory full of mushrooms, flowers, stars, clouds, and other funky things, and then save them for use later in the game.