I'd like to see more dynamic features as far as overall morale of your armies when your denarii goes into the negative and you can't pay certain armies their upkeep. Say decrease pay or no pay, some legions muntineed because of this.
That would kill the A.I. more than the human. Just like civil wars in MTW. You have to face a gargantuan empire only to watch it crumble into pieces after a few defeats.

Also if the troops were more expendable, such as having an army of 30-40 thousand on average then there's more room for attririon. Instead attrition in a 2000 man army woul be a potential pain in the butt. Especially if you have a mixed/blanced army and trying to merge.

I have no problem with attrition on besiegers just like in MTW. As of now only the besieged take casualties.