I got a suggestion for some more Research stuff. But I have no idea if it's going a little over the top. I tried to make it as balanced as possible.

[hider=Summoning]
Summon Wisps – Nobody knows what Wisps are. There have certainly been theories. But for now, nobody knows. Wisps look like small orbs of light to those who summoned them. If successfully summoned, a Wisp master can use wisps as his ears and eyes. However, fellow Magi can easily sense the wisp’s presence, and destroy it with great ease. (From a tile where there is a War-Magi/Magi unit present, you can send a Wisp to a neighboring tile. If said tile has no War-Magi/Magi unit present, then you gain Basic Unit Info and Movement.)

Summon Lesser Elementals – (Requires at least 1 ‘Lesser Elemental Magic’) The earth obeys your command. The wind heeds your word. With the power of your school of magic, the elements take shape of a living creature. These beings march forward into battle. Protecting their master as they wreak havoc on the battlefield. (For each unit of Magi/War-Magi you can summon 1 conscript unit. The unit returns to dust after the battle)

Summon Steeds – (Requires ‘Summon Lesser Elementals’) Using your Cavalry’s original horses as base, your Magi can summon even stronger beasts. Hellhorses, or Steeds of Wind, it does not matter. Your horses are faster and more powerful. Greatly aiding their riders. (Negates Monster and Mythical Creature Taming effects if researched. Your Professional Cavalry have an advantage in combat)

Summon Elementals – (Requires ‘Summon Lesser Elementals’ and at least 1 ‘Greater Elemental Magic’) – Creatures rise from the nearby river, and a forest fire starts to walk. Your Magi have learned how to bind more of an element into a single form. Making them far more powerful. But at great cost. The focus required to substance an Elemental army requires the full attention of your magi. So they are unable to march with their puppets into combat. (Before a battle, you can trade in a Magi/War-Magi unit for a Professional Military Unit (No magi, mechanical or Arquebusiers unit). This summoned unit returns to dust after the battle and you can again use your Magi-unit.)

Summon Water Spirits – (Requires ‘Summon Elementals’ and ‘Lesser Water Magic’) Water takes a specific shape. That of a legendary water Nymph. These graceful, humanoid beings walk behind the line. Their touch sooths and heals wounds. (Your soldiers are harder to kill when your Magi/War-Magi are present) 

Ritual Summon Guardians – (Requires ‘Summon Elementals’) Runes were infused during a ceremony. Or the elements called during a ritual. Whatever you did, you made a contract with local dormant Elemental Spirits. In the events of invasion, they will march to aid you. (If one of your own tiles is being invaded, 10 Conscript Units or 4 Professional Military Unit (No magi, mechanical or Arquebusiers unit) will appear there. Even if no Magi or War-Magi units are present on said tile)

Summon Greater Elementals – (Requires ‘Summon Elementals’) A hurricane takes shape. A firestorm starts to move. A lake begins to walk. A mountain stirs. Great beings of Old are summoned once again to march for their new masters. Towering over mankind, they break lines and sow chaos among the enemies. After which, they once again descend into slumber. To be awakened by another master. (Your War-Magi can summon a Greater Elemental (On par of size and force of a Mechanical Walker). However, the War-Magi is rendered unusable until the end of the battle)

Unsummon – (Requires ‘Summon Greater Elementals’) Your Magi, or Summoners, have mastered the ancient art of Summoning. Even to such an extent that they can order Elementals who are not even summoned by them to go to sleep again. (All creatures summoned at the start of a battle by the enemy are removed again. Enemy Summoner Magi are returned to the battle. This does not affect Summon Steeds)
[/hider]

EDIT: If you want, I could also start some brainstorming on a more mechanical "skill tree".