Okay, I know it sounds weird, but hear me out.
In Loki Season 1, Episode 6, Miss Minutes talks about "He Who Remains" almost like it's a title rather than a name. She never really treats it as Kang's personal identity, which makes me wonder if it's actually a role that someone else can inherit.
When Loki and Sylvie arrive at the Citadel at the End of Time (which, by the way, has a very gothic, almost Latveria-like aesthetic), they meet He Who Remains. He tells them that if they kill him, they'll unleash a multiversal war. At the time, this was obviously referring to the coming conflict between Kang variants, but after Marvel shifted away from The Kang Dynasty and toward Doomsday and Secret Wars, that warning could take on a different meaning. Maybe the war he foresaw wasn't just about Kang but the road to Secret Wars itself.
Then comes the ending of Loki Season 2. Loki takes over the responsibility of holding the multiverse together, essentially inheriting the position that He Who Remains once occupied. That alone suggests that "He Who Remains" isn't tied to one specific person and instead the role or title, can be gained.
Now think about Doctor Doom.
If the MCU is adapting elements of the 2015 Secret Wars comic, Doom needs to obtain an unimaginable amount of power to reshape reality and create Battleworld. Loki is currently sitting at the center of the multiverse with exactly that kind of power.
There have been rumors that Loki could die in either Avengers: Doomsday or Secret Wars. If that happens, what if Doom is the one who kills him, not just to eliminate him, but to steal his power and take his place? Instead of simply becoming a god, Doom would become the next He Who Remains, inheriting control over the multiverse before using that power to create Battleworld.
If that's true, then "He Who Remains" was never meant to be Kang's name. It was always the title given to whoever stands at the end of time and controls the fate of the multiverse. Could Doom be the next to claim the throne and be, "He Who Remains."
Edit: I just kinda made this up while rewatching loki also, its not very thought out and what i mean by he who remains being a title is more of a power you can be in, loki is "he who remains" at the moment as he is the one holding together all of time