Well, one way to get at the problem might be to think about the kind of shooting you do with the AR you already have, and whether you might want add a different capability with the new upper. For example, you might want a longer range capability with the new one. Or if you are looking to hunt with it, what size animal, and does that require a heavier caliber? Do you want something that is more accurate for competition purposes with a stainless barrel, tighter rifling twist, and free floating handguards?
If you don't reload, then it makes sense to stick with ammo that is widely available--which seems like zero right now. You mentioned having a lot of .223 already, so that makes it practical for you to stick with that caliber.
I am intrigued by the .300 AAC BLK, but that is because it is a heavier round that uses the same magazines.