Well, he absolutely has to have about 10 minor magic items that don't actually stack with anything but he has fond memories tied to them. "Oh, the coral pendant? This old thing? Oh, it was from the shaman of the lizardfolk tribe I erased about 300 yrs ago. I finally figured out how to use fireball efficiently. sigh... Thanks for noticing! Oh, and, fireball!"
+1 for a cubic gate, a way to escape without teleport. That's kind of done, and gives a hook to a new adventure. Oh, hey, his phylactery is hidden on one of the planes! Best find it before he reforms. He absolutely has to have a killer staff or wand. Minions to respect the action economy, probably summoned or constructed. Artifact level crown that has a jewel that is magical in its own right that can be pried off and used. (The using the crown itself would be ... unwise.) I like the ring of spell turning, especially with the number of spell casters in 5e. Would that affect the spells used for a paladin's smite? That's only one target.