This would probably cause howls of disapproval but what if combat when initiated had a variable 'defender or attacker' condition. The army/armies with the most remaining total movements points is the attacker the the army/armies with the least remaining movement points are the defender. Now your army with '4 movement' that has cavalry has some additional mojo going on. But also ... if you can make it so a sortie uses all the movement points of any unit involved - the sortie actually gets flipped on it's head it is an 'attack' but from the role of defender?

I actually do like the idea but it is tough to implement with the way turns are played.
It also does not really fair well with mixed armies.

My point is that it should reward mixed armies. Sorry if I explained that poorly.

Example. 
Army A 2 scouts and 1 (light) cavalry - sorry unit name blanking at the moment
Army B 3 warriors

Army B attacks Army A at the start of the turn without neither army moving. 
Army B has a total movement of 4+4+4 vs 4+4+6. This combat will have Army A as the attacker.

Army B sieges Army A in a city. 
Army B has a total movement of 4+4+4 vs 4+4+6 ... but in a siege army A has 0 movement. 
Army B will always be the attacker - even if Army A sorties out against army B.
Creating a resulting sortie that if Army A loses, Army B wins the siege. 

Because presently sieging is a bit wobbly sometimes you can lose a sortie and still not lose a city. You opened the gates somewhere along the city perimeter to launch the attack from.

Fair points.