I feel capitals can be used in a couple ways, here are my suggestions:
1) If you take someone's capital if they don't take it back in a few turns they become under control of the person with the capital if the other person takes their capital,theycan trade the capital's back or keep
the enemy's as their own.
2)The capital's can also be used as an extra troop bonus,just a small bonus like 1-2 troops for help to take down your enemy's or help your allies.
I hope you enjoyed my suggestions :D