I've seen non-assassin champions (probably, because their default traits, lol) but all sovs I've seen always were assasin...

i noticed this aswell (version 1.1 and older)

it's facepalmworthy when i see magic oriented sovereigns like ceresa or procipine with path of the assassin. just why? personally i like to decide based on lore and personality of the heroes. i dont expect the ai to do that but please make it not pick assassin, everytime.

There could be a "leader personality" feature like in Civ. They can be randomized in an option.

Perhaps these AI are simply defaulting to assassin in lieu of reacting to the circumstances. Would be nice if they defaulted (unless the circumstances called for it) to a more fitting path. Warrior for Yithril, Mage for Resoln and Pariden, etc.

They already have a 'leader personality'. It's described in the Foreign Relations part of the Govern screen. Maybe they can just hardcode the selections of all the standard Sovs.

Any custom faction created after 1.02 (I think) gets possible personality traits assigned to them randomly.  You can alter these, of course.

How they play really depends on what traits the faction has.