They could make the arena there, as I said before. Ifrit can spawn a bowl of embers. Garuda a tonrado, moogle, idk tons of moogles. Also if you are to close to it you will die(on the outside). The battlefield will change, it would be amazing, it would still keep that instanced feel but it will be open world.
I won't lie, I wouldn't mind being able to witness someone's Garuda run in the game, rather than on You-Tube.

Maybe it can happen in the future, but I'm not expecting it for 2.0. Something like that would take some work and time, and I would like to have some unique fights in the open world, rather than just rehash instance like ones to be visible in the open world.

It'd be nice, but not a priority.