This is one case that is strongly supported by the background materials as well. The Lion has reportedly been sleeping at the centre of the Dark Angel's Rock for a long while. Possible beside Luther (or maybe not). The last we saw of the Lion was when he descended for his battle with Luther on the (then) surface of Caliban. Following its destruction, the Lion was nowhere to be seen. Luther whimpered something about the Watchers in the Dark having taken him.
Hence, there is no contradiction in the background materials about him coming back. I'm far more at ease with the Lion coming back than I was about Guilliman re-materializing after healing in a stasis field. Heck, I'd also be fine with the re-emergence of at least:
Vulkan (because, you know, he's immortal and all),
Alpharius (or Omegon) (or someone being Alpharius at any rate),
and any Daemon Primarch in general (because why aren't they attacking the rotten heart of the Imperium already?).
Can Alpharius Omegon please turn out to be loyal? You know, just to wrap up that story line. Pretty please? And the only person he told was someone else who went missing at a similar time? I don't know where I'm going with this really.
Bottom line, I feel re-invigorated by the new fluff that seems to be coming out recently. And I'm feeling very positive about the future of the hobby in the near and medium term! Long may it last!