Yeah I noticed this, mainly since it was discussed so extensively before I actually got to play the games. As for the Musketeers, while I pretty much knew the other things (other than the Dream World being apart of opposites), I didn't really pay attention to their back story, or think it even mattered. Makes sense, though! Something else, is also Opelucid City, which is old in one version and new in one version, connecting the old and the new together.
You've also got Lenora and her husband, which don't... really match up in a normal sense but sorta fit together in-game. Can't really think of anything else off the top of my head, though. I think there's really a whole lot of things we haven't thought about here when it comes to opposites, but yet I aim failing to think of them!