I like these 3 sets of ideas: Mahazel, Luckmann, and Evernoob's.
A modest merger (altered quoted ideas in bold) with my own crazy ideas:
Force Mana; +1 diplomacy per mana.
Dimensional Mana; +1

/city per mana.
Creation Mana; +1

/city per mana.
I like.
Force I - Magic Missile
The caster gets "Magic Missile" promotion (+10% strength, +10% defensive strike and +1 first strike per force mana, as a "force affinity") until next combat.
Force II - Force Shield
'Temp building'-spell. Units in city are immune to collateral damage & immune to one first strike (per attacker). City takes 1/2 less bombard damage. Ends if the caster unit leaves the city.
Force III-Levitating Citadel
Rather than 'force construction', this converts one citadel tile into a new unit, the Sky Citadel. Citadel tile is destroyed in the process. Sky Citadel has 1 move per turn, unlimited by terrain and can cross water. Sky Citadel's offensive strength is 1/2 the offensive strength of all missile and siege weapons loaded on it. Sky Citadel can carry 5 units.
Melee units attack Sky Citadel at -100%. Missile units attack citadel at -75%. Siege weapons attack citadel at -25%.Citadel can't be captured from a boarding party, unless attacked from another Sky Citadel or Kurioates' Sky Boat. Sky Citadel has a defensive strength of 10, but can't attack normally.
Sky Citadel can make one suicide bombardment (does 20% bombardment to a city, strength 10 attack, and causes 20% collateral damage to the stack)----destroys everything that was still in the Sky Citadel, however; Suicide attack is assumed to be against an adjacent city, otherwise assumed to be against the largest, adjacent stack of enemy troops.
Any units loaded in a Sky Citadel when it is destroyed/suicided are destroyed. Loaded/Unloaded units from a Sky Citadel can't move for 1 turn.
Force Tower
Yields a meteor-like ICBM. One use only. Can subsitute for another tower in the tower mastery victory condition, IF it's not fired. Appropriate tech?
Dimensional I -
Escape
Dimensional II- Transferance
The caster can channel resources from a specific point, straight to the civilization capital. Ends if the caster unit leaves the resource.
Can only be done: if standing in self/unclaimed/barbarian cultural tiles, or if in a declared enemy's (i.e. at war) tile. (Should make some interesting game play)
Dimensional III - Gate
Creates a "gate" at the point of the caster. Any unit in a city with a Obsidian Gate OR at the point of another Dimensional III caster w/ a "gate" can 'airlift' to a gate. Ends immediatly if the caster leaves the tile.
Dimensional Tower
Creation ups armageddon clock by 3. Provides one chaos mana.
Gate that each use generates a random number of armageddon points (1-5 points, acts a little like the Ragnarok wonder), but gates in generally better monsters than Sheaim portals do at low armaggedon levels. Also gates some non-Sheaim monsters, like ogres, dragons, and the odd, carryable, special item. Payoff to armaggedon points is somewhat random.
At high armaggedon levels, the gate fails permamently. At that point the tower provides one entropy mana in addition to the chaos mana.
Creation I - Conjuration
'Temp building'-spell. Adds +3

to the city it's in per level of channelling (+9 with Channelling 3). Ends if the caster leaves the city.
Adds +5

to a tile if standing outside of a city. Of course only a city actually working that tile will actually benefit, and the spell must be re-cast is the caster moves.
Creation II - Wondrous Warehouse
Temporary building, cast in a city. Acts as a 'third' granary type (20% food stockpiles saved, to a max of 60% now), also randomally generates +1 health OR +1 happiness each turn. Effects lost if caster moves.
Creation III - Wonderous Creation
Temporarily creates a single building in a city, randomally from a list. Building only remains while caster stays in the city. Building gives every unit produced henceforth (in that city) a (single) item that in turn grants a predetermined effect (Haste, Enchanted Blade, Blur). Every building is exclusive to all other buildings created by Wonderous Creation. Removes Wonderous Creation promotion from caster (can be re-picked). Spell takes 2 turns to cast
Some ideas:
Magic Sandalshop: boots of speed- -1 terrain cost
Enchanted Swordsmith: Free enchantment I effect.
Necklace of Spell Extension I.
Tower of Creation
+25% Hammers in city, -20% food stores per pop growth point in city, +1 happiness in all cities.
Every 20 turns, randomally picks one mana type to provide for the next 20 turns: either Earth, Water, or Nature.