Eonbane

Y'know if they made a sequel to TotK to complete the BotW/TotK thing as a trilogy.

I can see it now. Vaati could make an appearance here as the main antagonist of that game......as for how he could assume this role.....look no further than the Yiga Clan which were mere nuisances at best for our hero and mere scoundrels for most of the people of Hyrule. Here in this hypothetical instalment, Vaati could be a member of the Yiga Clan but be disillusioned how most Clan members are incompetent to semi incompetent at what they do and how they constantly fail at their mission to eliminate Link and Zelda.

In this game we could see a time skip where Vaati a member of the Yiga displays his talents and would be recognized by the clan at large, only to eventually gain enough support to usurp Kohga as it's leader.

During the prologue we would see the Yiga mine for remnants of the Demon King's power and when Vaati comes across a dubious shard that was once from the Secret Stone the Demon King once stole them corrupted. Vaati uses his exceptional sorcery to restore and augment it's power, allowing Vaati to assimilate the Demon King's power into his own. This would then see the Yiga being empowered enough to invade and conquer the other races of Hyrule alongside the scattered remains of the Demon King's army, this setting the stage of this third installment of this hypothetical trilogy.