There are several different sovereigns to choose from, but have you considered having different ENEMY sorcerer kings to choose from? (You would still only play against a single SK per game, but just like your sovereign he could be a different class).
It would be a neat avenue for replay-ability to be able to play against several enemy SK's that behave very differently and have different lieutenants. Some ideas:
-Inquisitor SK - spreads his own brand of twisted religion from a benevolent facade. Loves to cut deals with the player. Converts neutral units to his side. Builds monuments to himself after destroying shards, which increase his power. The player can destroy the monuments, which would be well defended. Controls religious zealots and holy units.
-Devourer SK - He is legion. Personally stomps around the map, killing and absorbing everything in his path. Becomes larger and more powerful with each enemy he dispatches. Killing him on the main map sets him back a little, but he respawns at his fortress. Will come to your cities and give you the choice to bribe him with higher doomsday counter or be eaten. Eats shards, which give him a big power boost.
-Necromancer SK - increasing necromatic power as the doomsday counter goes up. Eventually, any of your units that die will rise in place as undead to fight against you. Zombie apocalypse, medieval fantasy style.
-Demonologist SK - destroys a shard and opens a portal to hell in its place, summoning demons for him every few turns. The more portals that are open, the more powerful the demons coming through.