I'm pretty sure that Eternum Nocturne and Aether Wing Kayle are connected somehow. Here, some of their quotes. Try figuring out whose is whose:
"There will be no compromise."
"Fate cannot be averted."
"Their existence is a mistake."
"Transcend imperfection."
"They pose unacceptable risk."
"Fulfill destiny."
"Long have we waited."
"We shall restore."
"In time, all will understand."
"We transcend time."
"I see into infinity."
I'm still not super sure for some of these whose is whose. A lot of them are saying something that the other says but in a different way or use a specific tone (Kayle focuses on perfection through progress, Nocturne focuses on perfection through reversion) so I couldn't put them there but still, things like these overlap extremely closely.
"To rebuild we must first destroy."
The end of days draws near."
"What was, is, and will forever be."
"The first are we. The last are we."
"In time, all will understand."
"The future is perfection. It's a pity you will not live to see it."

Seems like to me that they're connected somehow and there's also some similarities with Kayle and Nocturne in and of themselves. They're both extraterrestrial (extradimensional even) forces who have entered into servitude to the League, both because of the League's actions indirectly.
I know that skins aren't implicitly lore but it shows some interesting channels for thought. Nocturne and Kayle both seem like agents of a fundamental force for order bent on bringing a new greatness for the universe from the current wreckage. Nocturne seems focused on the destruction aspect and Kayle on the rebuilding aspect.
What do you all think? I kinda wonder if this has anything to do with future developments of lore or tying this in to the odd activities of the League.

Bump.

An interesting thought, however both their background lore's seem almost mutually exclusive to one another, and Nocturne, to my understanding, doesn't seem so much as an extra-terrestrial as a manifestation of nightmares. I could see how one would twist that to fit, but it doesn't seem likely. The quotes however could definitely be a prelude to something proving everything I just said wrong.

I interpret Nocturne coming from an alternate but adjacent dimension.

for me is like this nocturne fuse with other nocturnes from others dimension, and that will explain why he talk with "we" like is were some kind of legion (the infamous demon)

It's not really base Noc, though, we're talking 'bout Eternum Noc here. Same thing, but different. Normal Nocturne is kind of a manifestation of nightmares, brought to live by an unknown force.
Eternum Nocturne is more of a time deity, maybe even the manifestation of time, given the fact that 'nothing can withstand the tooth of time'. It's certainly the kind of vibe you get from playing him, especially from those quotes.
The thing about him speaking with 'we' bugs me though, maybe there's an infinite amount of them, one for each possible timestream? Or maybe it's referring to the fact it's immortal as well as able to travel through time, also making a potentially infinte amount of him. I really don't know though :/

Either way, it's an interesting view, and something definitely worth exploring.

Eternum Nocturne is a reaper... (quotes give this away)...I suppose Kayle could be a forerunner or Protoss

Oh, yeah. This reminds me of something fairly important. Eternum Nocturne was called "Dark Templar Nocturne" in development.
For those of you not familiar with the crusades, Assassin's Creed, or anything written by Dan Brown, the Knights Templar were a military-religious organization established in Jerusalem after the First Crusade. They were officially disbanded in the early 1300's (and by "officially disbanded" I mean destroyed) but is believed by some to have survived, possibly via the Freemasons. Okay, so by "some" I actually meant "the insane".
Not implying that Riot's lore writers are lunatics by any means.
But the name "Dark Templar" has some significance here because of the connotations of the Templars being the military-religious organization. Because hey, who in the League is our Templar? The champion with skill names like "Holy Fervor", "Reckoning", and "Righteous Fury" sounds like a decent bet. Yeah, that's Kayle.

I thought that Ezreal's Pulsefire skin was to be the antithesis of Nocturne Eternum skin?

I always thought it was Pulsefire versus Battlecast.