Hmm that's true. The only reason I started boxing was because I was bored with the current WoD content. I'm far from bored in Legion, and have a lot of content still to explore.
But, if I am to be challenged enough playing 1 character, I cant see that with my BM hunter. That class is so easy, it's no problem keeping up an optimal dps rotation while at the same time dealing with any kind of encounter mechanic.. :P
Back in WoD I played 3 hunters before I expanded to five. That was kind of nice for the reasons you describe.