Actually, although Sekiro takes place in the Sengoku era which is just 50 years after medieval times, the Armored knight boss is wearing armor that comes from Europe tying in the medieval themes. Also inspired by Berserk with the inclusion of Wolf’s prosthetic arm which can be fitted with variations of the weaponry added to Guts’ prosthetic such as, the fire cracker (bomb), the canon, shiruken (arrows). Notably the themes of Berserk although subtle still persist as Guts and Wolf share the same goal, to save their loved ones after having been defeated in an arduous battle. Sekiro is technically a “nobody” only because of his title of Shinobi where he is essentially an assassin whose presence is mostly obscured by the enemy. Whilst Sekiro inverts the typical souls narrative of reclaiming the position in power in a “dying world” the end of Ashina, his main priority is to sever the ties of mortality and thus aids Kuro throughout the game in order to fulfill the request. One of the endings does revolve around fire after Sekiro becomes Shura and is engulfed in flame. The final boss of both Shura and Severance endings are husks of their former selves. Shura Isshin is at his lowest power level and near death, Sword Saint Isshin is the version of him after having succeeded in his Coup although badly injured after having lost vision in one eye and other bodily injuries that wouldn’t deem him in his prime.
Headcanon that Sekiro and Issihin were lovers (hence the long dialogues reminiscing over Sake), Isshin’s disguise as Tengu in order to conceal the fact Isshin couldn’t be seen with Sekiro as lovers (shirtless of course). Which makes the final battle even harder because the words “hesitation is defeat” is exactly what Isshin had told Wolf over drinks when Isshin was confessing his love for him. Hesitation to find love within Isshin, to stop the killing and to prevent shura.
Even under the golden sun, in Buddhism gold represents enlightenment, love, and purity, the clouds obscuring those parts of gold represent Sekiro’s demise and fall to Shura, but they also could be a metaphor for Wolf’s hesitation to actually love Isshin fully. He had to kill Emma because it was his way of fully abandoning that relationship with Isshin, his trust, his love gone but in the end Isshin says “I couldn’t bring myself to hate you”
I meeeaaan, Isshin was very close to Sekijo/Monkey/the Sculptor, just swap him for sekiro in your theory and it's pretty much canon already. We know Isshin adopted Genichiro and afaik we don't see any het partner implied for Isshin sooooooo...
It also already works with Isshin having to sacrifice & save Sekijo from Shura and abandon that relationship for his dreams of ruling the land - he chose his ambition and now in his old age reminisces about such things as, in the moment, hesitation is defeat. We already see the events of Sekijo's life repeated in Sekiro's, and he is very much a thematic mirror of the older shinobi.