I totally agree! I have been waiting for this update but it just seems like it's not coming. Along with this, though, I think there should also be an addition of Celestia and Wintertusk to the pet pavilion games.
I actually have a design for the celestia arena, it would be underwater, with big fancy glass domes like you saw in the preveiw pictures but never seemed to make it into the game. There would be one in the center just barley big enough for the duel circle, surounded by smaller ones with glass tunnels conecting them for the spectators. The exit to the lobby is a large hatchway with a metal tunnel behind it that goes into some rocks. maybie there would be some small crabs between the duel circle and spectator area that are watching the duel.