Let's talk pairings - Comet is a grrrreat mold, but I think it's reasonable to expect that a lot of people are going to want to pair them with something to add an OS or US bookend.
KC Roc or other Rancho Roc seems like a popular choice, and for good reason; comets fly a lot like a well seasoned roc. For me however, I'm not a big fan of the flight of a fresh roc, just doesn't do it for me.
Another option not too far off on the spectrum would be a Wasp or a Roc3 - very similar in the hand to me, and similar stability. Wasps edge most Roc3s out in stability, but they both share the beaded profile and relative flatness.
Other than those, you enter the realm of more modern, sleek S/OS mids. Truths of all kinds seem a little long and fast to really pair with comets, unless you really love mids and want to throw them as a complement rather than more of a "Comet OS". I have considered something like a Pine, as it seems to hit the sweet spot of comfy to forehand (for me, anyway) while not being too hoggish and OS. I would consider a gyro mold too - I'd probably skip the Vector in favor of a wasp, but the matrix is worthy of consideration here too.
I do bag a Mortar, and I think bagging a true OS mid next to a comet is far enough removed to leave a large gap to fill with a S/OS mid. At this time, I'm skipping the S/OS mid pairing, and throwing a fresh X comet on hyzer or powering down a neutral fairway. In the future I'll likely throw a Wasp in the bag.
As far as US bookends go, I find most "US" mids to be very different than the comet, particularly in the realm of speed sensitivity. I've been looking at throwing a star mirage as my understable complement, as it encroaches on mid distances while not falling out of the sky when thrown with similar speed to a powered down mid shot.