Lorewise, is it explained how we can return to those zones? (EW spoilers)

[Cleretic]

Elpis' explanation amounts to 'yeah that portal's still open'; it's not much of an explanation, but also doesn't need to be. Ultima Thule's is a bit more involved.

Since you asked roleplayers, though... I don't think I know a single person who has decided to set a scene in either of those places, and I know quite a lot of them as an RP venue owner. Granted I don't make a habit of asking, but most people's character stories just don't really stretch out that far, people generally prefer more... well, terrestrial plotlines. I've seen one person whose description says they're playing an Omicron, but I haven't talked to them enough to find out what their angle is.