1. Fire Storm, a strategic spell that destroys buildings is a good one. I would make it really effective against wooden buildings and less effective against stone buildings. That said, I would suggest two spells; Minor Fire Storm (20 Mana) and Major Fire Storm (30 Mana) with a range of 5.
2. Another factor to consider for firespells is linking the effectiveness of some firespells to the terrain;
Example: Wild Fire which is effective in grassy and forested grass terrain does not work in desert/rocky terrain where there are no combustibles.
Likewise water spells should be more effective in terrain that is close to a water source, and less effective in terrain (deserts) without any water.
Example: A tactical map has a lake or river. A water spell caster would be more effective on such a map. Likeswise, a city that is close to water would benefit more from the spells of a water spell caster. In fact, for a city based close to water I could see a tactical spell of summon Mermen - these warriors would return to the sea/river after a battle.
3. Elemental specific buildings
- Guild of Fire Mages = +25% bonus to research of fire elemental spells
- Guild of Air Mages = +25% bonus to research of air elemental spells
- Guild of Earth Mages = +25% bonus to research of earth elemental spells
- Guild of Water Mages = +25% bonus to research of water elemental spells
- Necromancers Guild = +25% bonus to research of necromantic spells, reduces population growth due to research requirements (fresh dead bodies).
Each city can support the guild of only one elemental house due to inter-guild rivalries. Thus a player must choose - do I focus on Fire, Air, Earth or Water magic, the evil Necromantic?
Destroying a constructed guild house to build another, also causes surrounding buildings to be destroyed.
Exampe: A sovereign orders the destruction of the Guild of Fire Mages and the guild members fight back, causing 4 (1 to 8) surrounding buildings to be destroyed along with the guild house.
Exampe: A sovereign orders the destruction of the Guild of Water Mages and the guild members fight back, causing 8 (1 to 8) surrounding buildings to be destroyed along with the guild house.
A guild house may occassionally (5% per turn) ask for gold from the sovereign to finance their research. If no gold is contributed the research bonus stops