Unless they pull an Amon and resurrect Solus (the 1st Garlean Emperor) I think it's safe to assume that Garlemald's power is slowly weakening. Assuming we'll kill another Legatus and eventually Varis then that's pretty much the final head in the coffin.

I think Solus had many of his family members killed, so there aren't too many royals left, if at all. Then there was that brief civil war, razing of Doma (resource) building of Gration which cost ALOT.

Based on what little we know, I am very suspicious of Sharlayan