if the manga didn't stop being published the mangaka can just make a original end to bring closure to it.
Take "https://mangadex.org/title/3440/mm", the true author died and the manga artist drew the final chapters for a while after it so to finish it, I looked at the raws, it wasn't the best of endings but it didn't just stop there.
Is a similar case of Zero no Tsukaima, author died (R.I.P) but as it was too huge of a serie the editor made someone else complete it with ideas and stuff left behind, the fans deserved it (I'm not actually a big fan of ZnK so I don't know if it was a good ending or not).