So if I'm piecing everything right...
The body is the hero of Hyrule from 10,000 years ago who wielded the master sword to defeat Ganon. And dark energy from Ganon's defeat is seeping into him taking over/reviving his body. Ganon in human form = Ganondorf.
So wouldn't that might suggest the energy hitting Link right now, might be the Original Heroes soul attaching himself to Link for some cool equipment attachment all while being Link's companion for the game like that concept art someone shared suggests?
Well, as I said, I personally think the corpse is just straight up Ganon/Ganondorf. No idea about who the hand/arm is that's "plugging" the malice pouring out from him though. It could be a previous Link, but I'd probably bet on it just being an original character.
I do get where some people are coming from with the mural thing though, but I think it's more of an artistic choice from the designers where they're trying to mimic primitive art styles which were limited in their color palettes. After all, Zelda's skin color in that mural is also red, but we know she doesn't have red skin. I just think it's an artistic choice.
I think some people are getting the history we know from BotW a little mixed up though. Based on what's said in the game, the return of Calamity Ganon 100 years prior to Breath of the Wild is
at least the
third time Calamity Ganon has shown up in Hyrule. If you recall the story, it's said 10,000 years ago they basically had the Divine Beasts and Guardians ready for Ganon before he returned, and he was sealed away like he was a joke. They knew he was coming, because Calamity Ganon had already attacked further in the past at one point. The stuff in the murals you see throughout the game is from at least the
second time Calamity Ganon showed up, because they knew about him even back then. So it couldn't have been the event that created Calamity Ganon, but it was the first time Sheikah Tech was used on him, which is why 100 years ago didn't go so well, since Ganon learned from it and was expecting it.
This is more or less explained in the Impa cutscene:
The blue hand/arm looks to me like it's the reason Calamity Ganon exists, since Calamity Ganon is the result of Ganondorf. The blue hand/arm is either the cause of the outpouring of malice or perhaps Ganondorf went too far with something and the arm is holding back the result of that. Whoever's arm it is, it has likely been there for a lot longer than 10,000 years. I think the story will uncover some things about "Ganon's transformation into malice", as the King of Hyrule put it (Cutscene before you get the paraglider).
Part of me wonders if we'll have a Light World/Dark World dynamic. Where you have to navigate both of them at the same time to accomplish a goal, Zelda in one and Link in the other.
God, I wish we had some actual information on this game. I want to know more so badly.
I was literally thinking the same thing on this too, where one world influences the other just like the Light/Dark worlds, and you have to play as both in each world to progress in the game.
I feel Zelda may be playable in some form because of her shorter hair, as weird as that sounds. I feel it makes it easier for 3D modelers and animators to work with shorter hair when it comes to armor systems like Breath of the Wild since you don't know what armor combination a player might select. It's not impossible of course, it's just easier. Shorter hair would essentially make her model mechanics work much more similar to Link this way.
If she's not playable... Then Nintendo is pretty deaf. People made a lot of noise about wanting to play as a female character, either as female Link or Zelda, back when BotW was still being shown at events...