So I think I am going to use the evil wizard-king as the main villain. He manipulated the Royals for decades and corrupted the whole system until finally eliminating them -- except for the last heir who he keeps in thrall. He is a wicked, selfish, nasty, vindictive and capricious monster of a man, mad with power (temporal and arcane). All that is left in positions of authority in the realm are apologists and sycophants and cowards. I will let the players decide what horrors the wizard-king heaped on their loved ones or communities that has driven the PCs to swear revenge, and there will be lots of factions (both good and evil) they might find support from.
I will be running this in D&D 2024, so once the MM drops I will be able to build an appropriately powerful statblock for them to be forced to wait until late game to face. It looks like I am going to have to lean on experience and 3rd party products to build my sandbox, though, but I have time yet.