If they pull an Yhwach, Kaguya or anything like that it would ruin the entire story. They better just give us a Muzan backstory enlightment before he dies - we need to know who, how and why that doctor gave him a demon transformation medicine - and that's it since it's in the final arc and battle.
As far as we know, Nezuko is human again. The only demon that could walk on the sun and achieved Muzan's ambition is now human again. What if Muzan, just as he is about to die, do something that involves her? We have to wait.
Also I don't think Tanjiro need to be the greatest for it to end perfectly. Just like you, I'de be happy to see a different conclusion.