From a game statistics standpoint who would want to be a fighter regent?
After all a thief gets RP from law, guilds, and trade routes. Not to
mention thier free esionage action.
And priests; free agitate action and RP from law and temples, along with
realm spells.
Wizards have realm spells too, and special holdings that can't be
contested by military force.

In the warrior family there are also less things
Rangers have law and 1/2 Guilds
Paladins have law and 1/2 temples

Bards are not typically regents so forget them.

Any class can get warrior proficencies (MAYBEE A Little more expensive
but they can) and the battle benifits from them.

Maybee I am missing something but I am thinking about giving other
classes some benifits.

Perhaps

Fighter: Cheaper to muster amries/move troops? Free declare war action?
Cheaper to Fortify?

Paladin (can these guys cast realm spells?): 1/2 benifits of fighter,
bonuses to Agitate actions (priest part)

Ranger (they worship Erik right? Do they get realm spells, in any way?):
I can't think of anything now.