I thought about this for a while because a lot of them have had their back story fleshed out fairly well in flashbacks or their series is ongoing so we may yet see. So I finally settled in the following people. No reason for Luka above other than I like the pic lol.
We don't get much about her other than she's an oracle, a badass, and cute as hell. It would be nice to find out how she became an Oracle & got the name Rory the Reaper.
I would love to see a series of how he became such a badass. We get little pieces thrown in here and there but not enough to satisfy us. I mean it's 600+ chapters in and we just now get a peek at his bankai.
The second season left us hanging. Who exactly is Yato and why doesn't he use his real name? I don't see Light anywhere lol. It's revealed he's been around for a long time, so let us in on his past. I'm sure we'll find out soon tho since Noragami has been such a hit.