Tbf, i never thought Turalyon deserves to be villain material. He is literal hero and good guy back in WC2 story and following. Like with no flaws Arthas had. He was basically Anduin before Anduin.
Mind you, not good guy as like "Devoted to Light", but genuinely good person. Realistically it would be impossible for him to become fanatic being that good.
On other hand, he literally lived for thousand(s) years, fighting Legion. This can change people. But when we finally met Turalyon he still was a good guy even though his bond with Xera was obvious.
He is still devoted to his wife who is the bringer of Light's antipod.
He is still good guy.
Its honestly hard to turn him into not boring villain. Hence why i doubt Danuser will try to do it.
Turalyon on other hand may become nice anti-hero, who helped the Light but realized his mistakes and try to fix them.
I would even emphasize on his similarity to Illidan who he hated in Legion - the person who blindly followed Legion just to realize his mistake and then putting everything to fix it.
As for the who Stormwind throne belongs to - i doubt Turalyon will be the villain usurper. It simply doesnt fit him.
Id be more happy with Sabellian-Wrathion type of dynamic between older, way more experienced Turalyon and young Anduin. Like, Turalyon would tell Anduin he is too bratty to be the king, so the theme of Dragonflight would be both Wrathion and Anduin trying to grow into Sabellian and Turalyon shoes.