I think it would be great if influence would affect a city's defenses.
- Rallying Cry. Sway the locals to your cause. This adds a minor bonus to a city defender's attack/defense/hp.
- Last Stand. This adds a moderate bonus to a city defender's attack/defense/hp, and is castable/usable once per city.
The more that I think about it, the more I think that influence could be used to implement espionage.
- Desertion. An opponent within your borders has random units leave his party or return to his nearest town.
- Safety in Numbers. Cast on a unit (sovereign/champion) or party. This inflates the party's combat rating for a duration.
- Natural Diplomacy. Wandering creature wanders away from your borders.
- Natural Guidance. Wandering creature moves toward an opponent. The player pick's the opponent or opposing city, and the creature's liklihood to reach its destination is affected by the spell caster's level, creature's type and level.
Edit: 'Rumored Threat' renamed 'Safety in Numbers'.