It was suggested a while ago that Marnûkh will be the connective tissue between storylines like the mithril was in S1. It could be as simple as the mithril being part of the forging process of the Three and now Marnûkh is leading Galadriel and Arondir on a quest to investigate Sauron's forging the One, but I think it could also be thematic.
The showrunners have alluded that the main theme represented by Marnûkh is whether orcs are capable of redemption, which calls their natures into question. They were perverted in such a way that they became so far from what Eru intended for all living beings to the point that they're deemed something entirely 'other' and wrong. Marnûkh having distinct personality traits, individuality, desires to create art, etc. all humanise him in a way that's deeply unsettling to the elves because it challenges this perception of otherness.
We know that Sauron goes to Simon Pegg for help mastering the fires of the volcano, but also from the snippet of dialogue in the creature feature that they talk about the differing ways in which they've chosen to cloak their souls (basically calling Sauron a pretty boy lol). Usually, the forms Sauron takes are tactically designed to manipulate certain people. They don't reflect what he is, only what he wants other people to see, but there's also this element of him literally slipping inside the skins of these deceptions and getting so deeply into character that he swallows some of his own performance, almost like a method actor, but regurgitated through his own distorted lens—like his genuine belief that his vision for Middle-Earth is for its own good despite the obvious cognitive dissonance between this seemingly altruistic motive and violent method ("You really are the Great Deceiver, you can even deceive yourself")
I think the thematic connection between Marnukh and Sauron has to do with Sauron's nature as a deceiver, specifically in terms of being dishonest with himself, being what holds him back from successfully forging the One Ring. Charlie said his current look is essentially a corroded version of his Annatar form, which echoes the state of mental disrepair we saw Celebrimbor fall into and suggests that he’s still using his forging techniques, which came from a place of genuinity. But this method led to the creation of the Seven and the Nine because they’re merely perversions of the Three in the same way the orcs are twisted copies of Eru’s children—lacking all the facilities of the original creations, inherently flawed in design and subservient by nature, none of which are qualities Sauron is intending for the One, which is why the results aren’t the same.
In contrast, the balrog’s chosen visage is in perfect harmony with what his soul contains. Maybe this realisation will force Sauron on a journey of interiority (possibly manifested by hallucinations of these subconscious self-fabrications, like Celebrimbor or others he’s deceived?), and this confrontation with his own soul is a cinematic visualisation of him pouring his fëa into the ring to bind it, and his ascension to the true Dark Lord of the Second Age?
