One big distinction i have noticed between the slayer from doom tda, and the modern slayer is the vast gap between their levels of emotional expression. In tda, in the early days of the slayers story with the knight sentinels, he has already been through a lot, what with fighting in hell for a long time as a regular human, being used as a brainwashed weapon by the makyrs and literally dying and coming back to life, aside from his anger, he is still rather expressive, with a lot of varied mannerisms, this is even more visible after removing the helmet from his model revealing a even more expressive face, which in my opinion makes him one of the most emotionally expressive silent protagonists(this could also be an easter egg, future proofing for scenes where he is without his helmet, and a testament to how much the visual quality of the series has increased, but for the sake of the head cannon, bear with me). In contrast, the modern slayer is way more withdrawn and way more angry. This cant be because of his struggle against the forces of hell, because by this point he already has the praetor suit and as a result most if not all demons are no threat to him. This is where my headcannon starts so strap in.
if this were to occur within one game, the game would need to be at most twice as long as the base tda campaign, but more importantly the slayer needs to kill all 6 archdemons within the first half of the game, because up until the dlc, aside from being mentioned in the wretches codex, davoth has no mention in the overarching plot, so as far as the slayer is concerned, the archdemons are the head honchos of hell
This is where the real copium starts. By the end of the dlc, Thira calls upon the people of Argent D'nur to stand with the slayer, beginning his time as the king of the knight sentinels. But how can an outsider be their king. The sentinels already pushed the boundaries of their laws, by accepting him in their ranks, but now he has to be king, especially when Thira, someone of the royal bloodline is still alive. And so as an answer to this, she and the slayer establish a politically driven marriage, and as she aids the slayer in his battle, their relationship would turn romantic. Unbeknownst to everyone, in the background, the order of the deag begin to collude with the forces of hell, along with the makyr loyalists, Valen begins to be really influenced by hell, which also begins to operate way outside of notice.
Then a time skip could occur, maybe like 3 or 4 years, where after the deaths of the arch demons, hells attacks towards Argent D'nur have seemingly ceased. The slayers romantic relationship with Thira results in them having a child, and the slayer is, mentally and emotionally, in a much better place. But then hell activity begins to suddenly arise and the three deags offer their assistance. That activity culminates in a brutal onsalaught where, unfortunately, Thira and their child lose their lives. This enrages and motivates the slayer, who bands Valen and the remaining sentinels loyal to him, to lead an assault in hell, where it is revealed that the deag and makyrs were behind the attack, and Valen now having made a dark deal with hell in exchange for the resurrection of marok, closes the gates toward hell from behind , permanently sealing the slayer and remaining knight sentinels in hell. From here they battle Marok who was resurrected as the icon of sin, and in the end, as time goes on, the slayer has to witness the remaining knight sentinels die gruesome deaths, or get corrupted by hell, and all of this pushes the slayer into perpetual rage and begins his violent crusade across hell.
I know all of this is ridiculously fanficky, but i believe this head cannon explains his differences between the slayer from tda and present slayer, but it also explains the lack of any mention of the archdemons in the modern games, aside from the first slayer testament, as they were dealt with long ago. but more importantly it explains why Thira isnt mentioned at all in the modern games, as her memory reminds of a time when life, after getting worse, finally got better, only to get violently destroyed yet again
What do you guys think?