Your mention of Demacia kinda reminds me that Demacia has a weird patch to its visual consistency. Around the time Jarvan came out, further Demacian champions didn't look very Demacian at all, at least to me. Shyvana, Vayne, Fiora, and Jarvan himself, do their designs scream Demacia to you as profusely as the designs of Garen, Lux, Xin Zhao and Galio? It wasn't until Quinn and Garen's VU that they seemed to be re-embracing that old Demacian aesthetic, but even then we have Lucian, who also looks kinda removed from Demacia.
Something to consider is that League's visual style really became solidly grounded around 2012 for a lot things. The likes of Hecarim, Ziggs, Syndra, Jayce and Darius set the bar by which the standard for their nations are held visually. You look at these champions and you can instantly say: That's the Shadow Isles or, They're a Yordle! These visuals were later embraced and adapted on by further champions later down the road:
- From
Jayce we got
Vi
- From
Hecarim we got
Thresh and
Karthus
- From
Ziggs and
Lulu we got
Heimerdinger and
Gnar
- From
Syndra and
Zed we got
Karma and
Yasuo
- From
Darius and
Katarina we now have
Sion
This is something we also saw for Freljord and Shurima as the design principles set by the Howling Abyss, Sejuani, Trundle and Lissandra helped to conceive Braum, and the design principles set by Sivir and Nasus helped bring about Azir, Renekton and Cassiopeia, plus all the Shurima related content that came with it.
With Sion looking as he does now, I think it seems safe to say that the visual style of Noxus has been set. It was set by Darius before, met by Katarina with her VU, and now Sion meets the bar with them. Reds and blacks, cold dark steel, spikes, skulls, power and brutality. That's Noxus, at least as far as I can see from this. If anything, the lack of visual cohesion with the older champions is because the older champions of League were not made with that cohesion included as a paramount principle of their design. Going forward we'll see champions (provided they come from established areas with established identities) abide to the aesthetic and style set by other more recent champions, going back no further than early 2012.
At least that's how I see it. Hope I explained that well!