I do remember them being in early concept art too. I guess there was supposed to be more going on there in terms of story beyond the Noct/Luna and Ignis stuff. But Sapphirus being an Altissian Imperial makes some sense to me, or at least it would've been cool seeing as there aren't really any cool warrior characters representing that nation, like Aranea does for the Niffs and Ravus for Tenebrae. Also wondered about what the hell Solheim was going to be since they refer to it as if it had been conquered by Niflheim in the Leviathan trailer? My far-out guess is maybe it was a ruined civilisation but not as ancient as it ended up being XV, because again, having a fourth nation in the story would've overcomplicated things.
I want to preface this by saying Ardyn's story is one of my favorite things in XV and something I wouldn't change personally. BUT a part of me thinks that maaybe he was supposed to be from Solheim, in fact back when his character's past was still a little murky I thought he might've been from Solheim before founding Lucis, but it turns out Solheim is way older than Lucis or Ardyn in XV. Again, baseless but Ardyn working with Niflheim and being from Solheim would've made some sense since the nations were always clearly going to be associated in some way, hence the names.
The most obvious one though is his name, and the names in XV are significant,
ardēns,
ardentis:
- burning, fiery
- shining, brilliant
- eager, ardent, passionate
So his name is associated with fire, Solheim is derived from the Sun. And less reliable sources claimed it was going to look middle-eastern, which reflects some old concept art of Ardyn in his modern clothes surrounded by what looks like an Arabic-inspired people. Also, Ardyn does have a red armiger which I don't think is ever really explained in XV despite him being of Lucian blood, and if Solheim did have a crystal as stated in said trailer, the red/fire color scheme would fit, along with the fact he has control over Ifrit. So if Leviathan was some kind of summon/protector of Altissia, Ifrit could've been that for Solheim.
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Again, could be completely wrong but I did notice his name and the names in XV usually do tell you something about the character or their lives. Ardyn was a passionate man so it could simply mean that really. But because so little was known about what Solheim was as the story evolved from Versus to XV, it does make me speculate. I think Ardyn being a Lucis Caelum makes the conflict more personal to Noctis as opposed to him being from some distant fallen nation. But I can imagine a version where Ardyn was betrayed by Solheim's crystal instead, it would justify the existence of a fourth nation in the plot. If they did remove Solheim as an active crystal nation, I can understand why, would've been too much to cover in a short time too.
I hope all this doesn't sound too crazy but it's just a thought I had. But I'm glad they ended up with what they did, Ardyn's one my favorite FF characters/villains.