Well, every sovereign can make a great spellcaster, but if you don't have summoner trait is not as good to specialize in those (sure can grab one or two for utility / big hits absorb like you know that ogre / troll is going to use his 1 shot mace skill and you bait it with the summon), the problem is making them a different class but still having good magic access (since is world of Elemental).
Playing Tarth on ridiculous now, made it to half of faction power list in early game, got 2 fortresses 1 conclave and 2 towns with plenty of resources so far, no AI really hating on me (I even demanded tribute from 2 empires), and sovereign is assassin, but so far up to level 7 I have yet to pick a single skill from Assassin tree as I wanted to get full potential on her (since am using a 2nd hero in her group, which is a life specialized mage on healing, otherwise things simply wouldn't really work with a melee assassin) and the lightning tree level 4 (so she has better mobility as it more easily accessible the teleport from Air rank 3 than shadow switch which is deep in assassin tree).