Brilliant idea Gandalf!
I would definitely go with a theme for the enemy alliance, like the dragons. Pick 3 different coloured dragons and match them up with nations that are in style with the dragons colour/attributes. Eg., red dragon = Abysia.
Then add 9 picks of the path that relates to the dragon's immunity. Red = 9 picks of fire magic, Green = 9 picks nature magic, etc. Since a pretender loses 1 point from each path if it is slain, it will be reduced in power each time it is slain, but not so much as a rainbow mage would suffer.
The AI will inadvertedly also make some nice moves by pure circumstance when you have a nation like Abysia and a fire immune, 9 fire picks, red dragon pretender (eg., when it eventually unleashes a fire storm, some of the units in it's army will just happen to have fire immunity, because that what Abysia is a bout). So despite the limited intelligence of the AI, it should prove interested for new players.
Make one nation based on fire, one on poison (C'tis or Machaka), and one on something else.. Perhaps an underwater nation would be a good idea. New players are generally hesitant to get their feet wet (after they have their first heavy cavalry section chewed to bits by a school of sharks.)
So maybe blue dragon and Atlantis as the third nation.
If you want Ermor as the antagonist, that could be a different (but similar) scenario. Ally Ermor with some of the other death themed nations and see if it works out (Carrion Woods, C'tis death theme?)..
/Rainbow