Easy Way: Energy Substitution Feat and modifying various visual effects.
(Note: normally Energy Sub requires 8 ranks of Knowlege(Arcana). Allow it to optionally require 8 ranks of Knowlege(Nature) for Fire, Cold, Electricity and Acid only.)
Also, consider allowing spells from other sources, like Magic of Faerun (MaoF) and Masters of the Wild (MotW).
So we have:
Summer (Fire)
Winter (Cold/Water)
Spring(Air/Electricity/Plant Growth)
Autumn(Earth/Acid/Decomposition)
Summer:
1: Faerie Fire, Cure Light Wounds, Handfire (MaoF)
2: Flame Blade, Produce Flame, Body of the Sun (MotW)
3: Cure Moderate Wounds, Protection from Elements, Bottle of Smoke (MotW)
4: Flame Strike!
5: Wall of Fire, Inferno (MaoF)
6: Fire Seeds
Winter:
1: Cure Light Wounds, Endure Elements
2: Barkskin (but LOOKS like icy skin), Creeping Cold (MotW)
3: Protection from Elements, Cure Moderate Wounds, Water Breathing, Standing Wave (MotW)
4: Quench, Sleet Storm, Blight (MotW), Waterball (MotW)
5: Ice Storm, Cloak of the Sea (MotW)
6: Antilife Shell, Wall of Ice, Drown (MaoF)
Spring:
1: Engangle, Obscuring Mist
2: Barkskin, Wood Shape, Briar Web (MotW), Master Air (MaoF), Cloudburst (MoaF)
3: Call Lightning, Spike Growth
4: Control Plants, Murderous Mist (MaoF), Wind At Back (MaoF), Miasma (MotW)
5: Control Winds, Wall of Thorns, Big Sky (MotW), Binding Winds (MaoF), Wind Tunnel (MaoF)
6: Transport via Plants, Greater Call Lightning (MotW)
Autumn:
1: Sandblast (MotW), Blinding Spittle (MaoF)
2: Barkskin (but LOOKS like rocky skin), One with the Land (MaoF), Decomposition (MotW)
3: Stone Shape, Meld into Stone, Poison, Contagion, Infestation of Maggots (MaoF), Quillfire (MaoF)
4: Rusting Grasp, Spike Stones, Blight (MotW)
5: Rock to Mud, Mud to Rock, Memory Rot (MaoF), Kiss of Death (MotW)
6: Wall of Stone, Stone Tell, Stonehold (MaoF), Crumble (MaoF), Contagious Touch (MotW)
-- Nifft