I appreciate the feedback, fellas. Upon reading all of the responses and doing some research of my own, I've arrived at the conclusion that if I were to purchase a Complete Upper, then it would most likely be chambered in 6.5 Grendel or 6.8 SPC.
However, the biggest takeaway that I have received from all that I've read is that practically every alternative chambering available for an AR-15 is a sidegrade tailored specifically towards a special purpose, but as someone who lacks any particular special purpose in mind, it would seem premature as well as entirely superfluous for me to invest in at this time. So for now, I'm just going to stick with .223/5.56 for the time being, and if I were looking to upgrade, then I'd look straight towards the AR-10.