I personally think that weather effects would be really cool. Like Lord reliant suggests, you could get a skill or ability as a channeler that might let you predict the weather (maybe the ones that choose the meteorician skill at the begining of the game will know what the next day's weather will be)
I see some ideas for spells too
Rain mastery > increases chance of rain in area + provides boosts to some water spells
Storm call > causes rain effect
Thunder strike > direct damage spell boosted by rainy weather
earth to mud > slows units crossing mud terrain > area effected increased by rain weather
Blizzard > causes heavy snow weather effect
freeze > has a % chance to freeze the foe solid > chance increased dramatically if in heavy snow
freezing wind > direct damage spell that does extra damage in snow
water bolt > direct damage spell that has an added freezing or slow effect when in snow, extra damage in rain
eternal night: Darkens the skies cooling the land> boost undead and certain other 'dark creatures'
light of day: holds the sun in the sky raising temperature> boosts all light and fire creatures, harms certain 'dark' creatures
the power of the sun: A unit ability that means it gains bonus during sunny days, but handicaps during cloudy , raining, or otherwise dark days
the power of the night: A unit ability that means it gains bonus during dark days, but handicaps during cloudy , raining, or otherwise sunny days
Fires of the sun: A direct damage spell that is boosted by daylight
Chill of darkness: A direct damage spell that is boosted by darkness
UPDATE: (more ideas)
Dread rain: Causes the magical storms stike fear into enemies and rally support for your cause: Adds a moral effects to rain weather
fire storms: Causes rain effect with high chance of lightning strike, except the lightning does fire damage rather than electric damage (ie, fire creatures could be immune)
drout: Drys the land > removes chance of rain effect and lowers production on certain tiles.