At this point, I only want to see despair.
Something along those lines:
Tanjiro kills what's left of the demon corps, Inosuke, Giyu, etc. They were the only person who had the slightest chance to defeat him, but now, nobody can.
Not even the sun, which he conquered. In the last page, we would see him walk away, with the sun above him walking away with a small text (like in chapters ending) saying something like this "Now that he has conquered the sun, and the demon corps are annilihated, who will stop him ?"
Of course, there's still Nezuko (family power), the fact that she could live "like a human" as a demon (implying Tanjiro could too), and his friends (power of the friendship)
But this sounds fucking cringe, I want to see them dead. That's what I would call a fucking great manga ending