Its kind of unclear what happens to the guy at the end of the story
Also we haven't had a shadow isle Champion since Yorick (Save some crazy Neeko theories)
Yorick is an old champion, but technically we haven't had a new Shadow Isles champion since Kalista. Now that Sylas is out, it's the second-longest region to go without a new champion, if I'm correct, with the first being Bandle City. However, since the new support is almost certainly going to be a Bandle City Yordle, that makes the Shadow Isles number one.
I'd love for Ledros to be a champion. It's possible that he's the new assassin coming after Mordekaiser, but if he was formerly a valiant knight I would guess he'd be a bruiser instead (and I'm still banking on the wristblade in Awaken as being a teaser for the new assassin).
He really sounds like champion material, though we know that the next champ is gonna be a whimsical supporter and they also have a non-humanoid champ and an assassin in the works(though these 2 could be one and the same) and Ledros doesn't look like he'd be any of these.
Another point that I'm wondering about is how he'd be differentiated from Mordekaiser in terms of themes.
I mean they always try to give each champ some unique theme to differentiate them from the others, but Ledros doesn't seem to have much going for him thematically except for being an undead warrior, which is kinda 
s thing already(granted Morde is getting a rework, but I doubt that'll change).
And even if they somehow give Ledros his own theme,having 2 releases of big armored undead guys within the same year sounds like something they'd like to avoid.
He has to have a connection with Morde. The story just lines up too much with the Morde teaser, the Hecarim/Kalista stories, and just overall champion schedule. Combine that he has an active part in the story with him setting off to go stop the black mist at the end of his story (I can't think of another character that has gotten that kind of ending). It feels really similar to Sylas's magehunter story too which backs up my theory.
If we're to believe Malicious Metal, that guy might be Morde. But he might be trolling us, so it's hard to say.
After rereading the story I'm pretty certain it's Mordekaiser, and they moved the timeline of the Ruination backwards a significant margin to make more sense. I mean, the Ruined King was said to be from a kingdom long since forgotten but the current Ruination happens AFTER proto-Noxus is formed, the Darkin fall, the Rune Wars, the Void, all that stuff and a kingdom that recent wouldn't exactly be long-since forgotten. In the story it's mentioned that Ledros has killed Hecarim several times and each time Hecarim comes back more nightmarish than the last, so here's my theory.
He leaves the Isles in search of a way to end the Ruination's curse, unbinds himself from the Isles by making his consciousness exist in his armor and his bones are where he resurrects (rather than the Mist), he creates the Immortal Bastion to house his bones and also serve as the stronghold for his empire which conquers the known world. He personally fights in thousands of battles to conquer the world and dies over and over, each time becoming more twisted and evil. Eventually he outright becomes hellbent on world domination, feeling a certain bitterness that he and Kalista never got to expand their relationship in life and blaming her for the Ruination in many ways. LeBlanc, either one of his liches or one of his advisers, betrays him by undoing his magic that keeps him independent and sending him back to the Mists, and after gaining power and becoming entirely enraged his "unfinished business" that lets him return is causing the destruction of LeBlanc and reclaiming his empire. The only thing I'm not sure of in this theory is my assertion that he eventually forgets Kalista or becomes bitter towards her and disregards that as his goal. The story makes it clear they had some major history, but it sounds like Kalista turned him down for marriage(?) to continue serving the crown, which she now regrets in undeath.
I dunno if I'm right on the Kalista thing, but just look at the way Ledros interacts with Thresh at the start of the story. The first time I read it I was like "oh cool, a pretty powerful undead who won't take Thresh's shit" but the second time it REALLY screams Mordekaiser to me. He gets insanely angry and his eyes flare with energy, he is a massive man who wears black armor, his face is devoid of form and only shows his eyes gleaming through his armor. Sure, he uses a sword now, but he throws his shield aside when he lets Kalista kill him and perhaps he leaves his sword behind as well? Maybe his broken sword is later traded to LeBlanc by Elise for more humans for Elise to sacrifice to the spider god, and maybe that sword is eventually reforged by the Noxians and given to Riven? Mordekaiser forgoes a shield because he becomes functionally unkillable, taking up a mace to deal maximum damage to the front line of his foes.
It's a theory on the forums so it's probably incorrect, like 90% of the things we guess doesn't turn out to be true. That being said, I did accurately guess Sylas a few weeks before his reveal down to the fact that petricite absorbs magic rather than nullifying it and that he uses the chains as weapons to steal magic from his enemies. I just hope that whatever Riot does to Mordekaiser, they keep as much of the current lore intact as they can because tons of champions have lore tying to the Mordekaiser as we know him. Timeline changes are meh, adding to his backstory and shifting a few events around aren't too bad, and giving him a reason to conquer the world is honestly cool, but all of that should add to the current story rather than change it entirely.
The theory is also built on the assumption that Malicious Metal isn't trolling us, but even without that Ledros is physically a lot like Mordekaiser.
