a) Charisma can buff troops, kinda like Bard's skills and leadership in the AOW series, tie it into that?
Maybe high Charisma scores could give a shot of +1 to move for stacks the leader is in?
c) Charisma could help with illusion/charming spells?
d) Maybe Charisma could also help buff a city a hero is in, if he is left in the city (production/prestige bonus?)
e) Charisma could give a bonus to combat speed for stacks the hero is in also?
I think all of those could help.