^ Exactly what I've said about captain forever now. It's impossible to balance the mode when the AI can't even use the tools it's given properly. Any changes based purely on captain is a waste of time and will ruin another mode while not really making much difference to captain in the long term.
Now if you were to balance modes seperately that would be great but that would be admitting the class system isn't doing it's job and that opens a whole can of worms there.
I've thought a little and I think it's also a problem that melee engagements resolve incredibly quickly compared to the other modes of combat. A slower resolving melee would lead to things like flanking maneuvers and missile support to become more important. TW have tried to make melee AI more defensive in the past but defending in mass combat without a shield is near impossible.
In the end it will porbably only be possible to prolong melee engagements when the AI learns to use distancing properly to stay out of harms way and even retreats when in trouble (I think another problem is that many Captain players are unaware that retreating is sometimes a possibility).
What captain really needs at the current moment is a tactic that counters a full rabble rush. I could even imagine that such tactics already exist but are too complicated to be reliably implemented with a team of randoms.
Edit: I've thought even more and think it's ok that some units just bring more raw power to the field than others. The ranged attacks of archers and the mobility of cavalry can to some degree compensate for that by giving their side a huge degree of map control and forcing the enemy to take disadvantageous engagements.
Two troops of archers in different positions for example can make the enemy team to either turn the back to one of them or split up and be defeated in detail. This only works however if they are threatening enough to bind more of the enemy army than the one troop space that they take from your own team.
I've thought even more and I think the two main criteria that unit balance should fulfill are the following:
1. Competitiveness: For every army composition A each faction should have at least one army composition B that is viable against A.
2. Unit diversity: For each unit type there should be at least one competitive army composition that contains this unit type. (We call an army composition A competitive if there is no other army composition B for the same faction that is just better than A in every situation.)