Adding too many different modes to a game makes it lose its focus. It's hard enough already to explain what Destiny is. Sure, space battles are cool the first few times, but a large percentage of the population who are hardcore fps fans would quit them pretty soon. I don't want to think about stats while choosing ship, and everyone would end up with the same ones. Or Bungie would have to design a whole ton of ships and other stuff to create and balance a space fight metagame. If you want to play a space battle sim, play a space battle sim.