My original intention was for spells to scale with intelligence and shards. However, I no longer think this is a good approach. Instead, I wish to make spells scale ONLY with shards. The reason for this is if you want two things to boost something, you have to make their individual contribution very small so that it doesn't become overpowering.
When it works:
Let's say I have a spell called Fireball. It does 6 damage, and each Fire shard increases damage by +2. In addition, For every 4 intelligence above 10, it adds +1 damage.
When it doesn't work:
Now we have a spell called Grace of Air. It gives you 1 combatspeed, and each Air Shard increases the speed bonus by +1. In addition, for every 10 intelligence above 15 it increases the amount by +1.
Then you start a game and you find one air shard and you decide to make a caster build, so you start pumping intelligence. You also go looking for another air shard, which happens to be nearby. You quickly gain a spell that gives +4 combat speed. Then you build a single archer unit, and you win the game.
Similarly, imagine stoneskin. It gives you +3 defense. And another +1 for an earth shard. Then another +1 for every 10 intelligence above 12. So now you quickly have a spell that gives you +5 defense. Doesn't sound so bad? Let's imagine a leather-wearing unit with 9 defense and 15 attack fighting an enemy with 15 attack as well (quarterstaff). Instead of having ~50% of its DPS negated by armor, it will now have ~80% damage negated.
What I'd like to do instead. Spells are boosted by shards only. The only function of Intelligence is to get less resists from offensive spells. I believe this will make the game more interesting, and bring shards and spells back into the gameplay.
Scalable strategic spells:
Inspiration: +1 tech knowledge. In addition, each shard of any type boosts tech production by +5% in the city
Nature's Bounty: +1 food, +0.5 per water shard
Invisible workers: +1 material/turn, each shard of any type boosts metal production by 5% in the city
Enchanted Hammers: 10% faster build speed in city, another +4% for each any shard
Regeneration: +2 health/turn in strategic mode, each Earth Shard gives +1 tactical health regen
Fertility: +1 prestige, +0.5 prestige for each shard of any type
Glyph of Life: +20 health, +5 health per Air shard
Arcane weapon: +3 attack, each any shard will give unit 2% chance to ignore armor on an attack
Stoneskin: +3 defense, another +1 per earth shard
Pillar of Flame: 2 damage, +4 fire per fire shard, 5 turn cooldown
Inferno: 10 damage to all units that enter the tile, another +5 per fire shard, lasts for 10 turns (10 turn cooldown)
Slow: For 3 seasons, enemy army loses 1 movement, +1 per water shard (6 turn cooldown)
Evade: +3 dodge +1 per air shard, now a continuous strategic enchantment
Scalable tactical spells:
Arcane Arrow: 6 damage +1 per any shard
Spell blast: 3x3 square, all units inside take 3 damage +0.5 per any shard
Guardian Aura: Removed
Touch of Entropy: 25 damage +5 per any shard, can only be used once per fight
Wind Shield: +6 ranged dodge
Haste: +1 combatspeed for 3 turns
Greater Haste: For all friendly units: +1 combatspeed, lasts 1 turn
Pull of the Earth: -1 combatspeed for 5 turns, -1 for each earth shard
Hail of Stones: 14+2 earthshard damage, for each point of dodge on enemy damage is reduced by -1
Storm: Random enemy suffers 5-8 damage, the range is increased by 1-3 for each air shard
Greater Evasion: Unit gains +5 dodge, +2 per air shard, for 2 enemy rounds
Burning Hands: 1 range, 8+4 fire shard damage
Flaming Dart: 5 range, 5+2 fireshard damage
Fireball: 3+1 fireshard damage, damage is multiplied by enemy troopcount (a 4-man unit would suffer *4 damage)
Firestorm: 10+5 fireshard damage, 1 radius
Grip of Winter: Enemy unit loses 3+1watershard dodge, has a 20%+5% watershard chance to lose their next turn
Blizzard: 7+3 watershard damage, 1 radius
Greater Grip of Winter: Each enemy has a 20%+5% watershard chance to lose their next turn
Blindness: Enemy unit suffers -3 dodge. In addition, accuracy is reduced by 1 per any shard.
Wither: Target instantly suffers 30%+3% per shard of its remaining health in damage. All damage is recovered in 4 turns
Contagion: All enemies suffer -2 damage per turn. On turn of casting, each enemy unit has a +4% per any shard chance to lose their turn
Heal: +6 health, +1 per any shard
New spell: Greater heal: All friendly units recover 2 (+1 any shard) health per turn for 3 turns
New spell: Seep wound: Enemy target suffers damage based on its lost health. For each any shard, target takes damage equal to 20% of the health it has already lost
I'm also toying with the idea of giving free access to each elemental book, as soon as a player controls that shard. So a fire shard would give you free access to the spellbook of fire.