Additionally, PoL has a pretty extensive backstory involving a dawn war between gods and primodials, Far Realm incursions (far realm artifact + elementals=demons, so conjure elemental gives you a lot more options), primal spirits that largely keep immortals, elementals, and far realmers off the surface of the planet and are the power source for druids, rangers, barbarians, etc. (note that they apparently have little to no ability to keep things out of the Underdark). It is also where the Feywild was first a big thing and the Shadowfell got its name.
There were a couple of classes that haven't been particularly well translated into 5e as well as some playable races that aren't playable currently. It certainly seems to me that they could fill in a UA with content.