While i've also been thinking how something like this could be realized, this idea suffer from some major flaws:
First off, engagements don't last for long, and outcome is - in almost all cases - predetermined by fighter armament and UFO type. Replacing a realt-time event with a turn based event drags out resolution (i.e. you have to invest more time) for a mostly predetermined outcome (at least if this is to reflect vanilla stats for fighters and UFOs).
Secondly, with the Battlescape consisting of nothing but empty space, what's the point, really? The whole point of battlescape gameplay in xcom is dealing with the the terrain and sightlines. Congratulations, you've replaced an 'out-out-place' real-time tactical event with a battlescape mission that lacks any tactical component. It's the same slugfest, but it's going to take more time.
Thirdly - and this is a personal opinion - i always saw air engagement as part of geoscape gameplay - which isn't turn based either. This mix of realt-time geoscape and turn-based strategy events was what set xcom apart from other titles. The aircombat is part of that experience - the whole point of is to be more hectic and fast-paced than the ground missions.
I like variety in my games. Replacing a game mechanic that's different with a game mechanic that's the same as a mechanic already in game takes away that variety.
I also like game mechanics to be engaging - air combat as it is in the game is engaging because it's designed to be. Granted, it's a slugfest, but it's over quick, it's kind of fun (at least for me) and it's different. Battlescape missions are designed around the terrain and in most cases you have to deal with mor than one enemy. You have to take into account multiple factors. That's what makes them fun and engaging. You plan to take away most of those factors. Granted, it would make for a somewhat different experience from ground-based engangements. It would also be worse.