This section is also spoilered because it's more or less me drawing Miquella and Godwyn into my pet theory that someone in the FromSoftware writing staff is a massive Bleach otaku. I won't go into too many details here but basically there's a lot of story parallels to Bleach lore, such as a society (Seireitei/Leyndell) dedicated to maintaining the cycle of reincarnation by using a special force of people (Soul Reapers of the Gotei 13/Hunters of Death) to maintain the order of life and death- namely by sending their soul to the Soul Society/Erdtree and having them reborn in the way they see as natural, all the while keeping their leader in the story/game secreted away and sealed so as to keep the current order of things going for as long as possible (The Soul King/Queen Marika)- hell, the God King is incased in something similar to Renalla's amber egg.
There's a lot more, but if that didn't convince you then have the hardest piece of evidence I have: Carian and Lunar Princess Ranni literally takes you on "A thousand year journey among the stars" as the child of (part of in Elden Ring's case) the god in charge of The Lands between, her body dead and announcing that you and her will return. A knight of her Carian filigree, Loretta, is dressed in sliver and blue if you alter the outfit back to its original state, and she's notable for using a blue bow that shoots magic arrows with the potential to fire several at the same time and many from the sky.
The character
Yhwach (Yeh-Ho-Vach as I say it, Yu-Ha-Ba-Ha in Japanese) is the son of the God-King and father of the Quincies, whom he gave is blood to, who use blue soul material Heilige Bogen (holy bows) to shoot arrows and can form bows which fire several shots at once and even rain arrows down from above by shooting upwards. Eventually his forces evolved beyond using mere bows into the Sternritter (star riders), a group of people with letter designation by Yhwach based on the unique power they gain that act as his personal army, who left the world after his body died when fighting later Gotei 13 leader Genryusai. Afterwards it took 900 years for his pulse to come back, 90 years for his intellect to return, with 9 years of reabsorbing the power of the impure Quincy to regain his strength. He then returned to try once again to conquer the world, which is the main conflict of the Thousand Year Blood War arc. Fun fact,
if you take the truth seeker preset in Elden ring and put a Carian sigil tattoo over the right eye you make a very bootleg version of Ywach's first Sternritter Lille Barro. Isn't that a coincidence?
"Spirelord you fucking moron, talk about Miquella and Godwyn not Marika and Ranni, I'm tired of reading so get to the point!" That setup was completely necessary because the next bit is the most retarded speculation that will make or break my 'Thousand Year Shattering' theory:
The DLC is going to be the Elden Ring equivalent of Sōsuke Aizen Vs Baraggan Louisenbairn down to the fucking powers the characters possess, except you might kill both of them. Miquella always seemed like a parallel to Aizen- hypnosis based power, betrayed their original life in pursuit of power (and a cure for Malenia) and thus sought out a way to evolve into a more powerful being (Aizen wanted power beyond Soul Reaper and Hollow with the Hogyoku/Miquella wanted to cure his demigodletness and become an Empyrean). To do so, Aizen went to Bleach's afterlife realm for hollows, Hueco Mundo, and made the original king Baraggan understand that he has no chance against him due to his shikai, demonstrating this by killing the army sent against him. The original scene ends before Baraggan fires a cero with Aizen taunting him looking like a Shinigami in his black clothing. This DLC might (KEY WORD BEING MIGHT) be Miquella doing the same, thinking his last chance at power is to become take Godwyn's place as the Prince of Death and restart his evolution- this time, instead of the woman in the tree, he will rise from the cocoon as the living Prince of Death.
Godwyn is similar to Baraggan Louisenbairn in a few ways: Baraggan's main power is to speed up and slow down time- there is no cure for Baraggan's Senesencia as it is a field of rapid aging that withers ones flesh and bone. While not directly comparable to deathblight, his skeletal appearance and ability to turn one into a skeleton is a (admittedly weak) connection. However, there IS an ability in his resurrección that is similar to deathblight: Respira, a black and purple, smokey miasma that rots and decays anything the air touches, which is Elden Ring's main method of inflicting deathblight- only difference is the smoke is black and
gold rather than purple. Another thing: I'm predicting that IF I'M RIGHT Godwyn will use a greataxe like Godrick that's been stylized similar to Godfrey's Greataxe because Baraggan ALSO uses a greataxe. The main difference between the two is that instead of a red gem that disappears after his transformation, I'm guessing the shape of the Death Prince Rune will be right in the middle, which shall expel deathblight when he holds it aloft. Both also would hold a major title in their realm as well, King of Hueco Mundo for Baraggan and Prince of Death for Godwyn, and if the DLC happens as I say they will both be defending their title against a powerful enemy.
Due to how Aizen operates in Bleach, there's also a chance that despite his powers not working on Those Who Live In Death, Miquella has or will hypnotize Godwyn in the same way that Aizen hypnotizes the Arrancar into compliance and his ex-subordinates/ex-superior. Having Miquella pull the strings the whole time and control Those Who Live In Death, including the Prince of Death himself, would fall more in line with his parallel character but would be inconsistent with game mechanics (which isn't impossible, it's a FromSoftware title) and also make Miquella being in the cocoon in Mohg's domain retroactively lamer because it means he lost nothing with his physical form, rather gaining a substantial advantage from being killed- meaning he had more value being dead than alive... Unless that's what they'd go with? He wasn't metamorphosing but rather killed himself to sustain the Haligtree for Malenia's sake? Like how Ichigo's dad sacrificed his powers to save his would-be wife Misaki I guess...?

Maybe if they want to limit how derivative it is but it'd undercut the reason for the big St. Trina in the Haligtree being his new body so IDK.
EDIT: There's a chance that Godwyn uses a different weapon. Because one could argue Baraggan's Axe is already in the game: The Winged Greathorn is shaped similar and has a miasma effect Weapon Art that decreases damage dealt and increases damage recieved for enemies caught within it. This makes me MORE confident if only because
its design is similar to that of a deathbird and thus having a corrupt version with deathblight seems like a logical next step, especially if the axe is designed after the other half of the Twinbird
.