One of the best parts of this game is Grand Arena in my opinion. This is what keeps me playing the game.
Quick background story - I joined the game because some close friends played and I thought I'd enjoy playing with them. Quickly I learned I was behind without a reasonable way of catching up to friends that has been playing for years.
A Grand Arena Sandbox solves both of those issues and lets me incentivize bringing in even more friends as players. A quick 4 person separate tournament among friends would be great for the game - adding practice for increasing the quality and ingenuity of high level play, and gamesmanship and comraderie for casual players hoping to enjoy the game with friends.
I'll probably never catch up with my friends to battle in their guild or to get into their arena tiers, but a side game with them - no rewards needed - would add a really nice multiplayer aspect for friends. I can't wait to challenge them with our own personal rules against them using their Galactic Legends in practice against me!