Note beforehand:  This system is probably really unlikely ot actually make it into the game for a number of reasons, it is more as an idea for people to think about and such.

This is an idea for creating and researching spells in a manner similar to unit customization, so instead of researching fully designed spells, magical research would involve researching different elements of a spell, than putting them together ot make a full one.  Elements researched would include things like direct effects (increased damage, defense, etc., the "meat" of a spell), durations (for buffs, debuffs, damage over time spells), AoE, possibly mana cost reducations, and other such effects.  These different elements would than be combined to produce the full spells that a sovereign of other caster would use.

An example of how this might work:

Using the apparent magicval divisions (earth, fire, water, air, life ,death), and following convention, fire spells were associated with offense and direct damage, they might start with two options for fire spells research:  direct damage (allowing them to create a direct damage, single target spell), or increasing unit offense (allowing them to cast a buff on a unit increasing its damage.)

Once, say, the direct damage spell was researched, it might open up, say, three options:  One would be to increase the damage (creating a more damaging spell), the other would provide a small AoE, the other would cause a Damage over time effect that was more mana efficient than direct damage.

If, say, the AoE were researched, it might open up further options, say, ncreasing the potential AoE size, or allowing overland spells.

Further research might allow targetting the ground (to, say, buff allies standing on a piece of terrain, or damage enemies standing on it), creating aftereffects from combat, continue ot improve the original damage, AoE, etc. abilities of spells, and other possible effects that I have not thought of.  the "offensive buff" tree could leed ot speed boosts, berserk type abilities, summoning, etc.

When the faction leader decided to actually create spells to cast, they could combine these different elements as they saw fit.  (for example, if, say, AoE and damage over time abilities were combined in a spell, the resulting spell would cause a damage over time effect on enemies in an area.  To get something equivalent to, say, a diablo 2 meteor, someone might combine ground targetting, direct damage, and AoE).

The mana cost of the spell could either be determined by directly adding mana costs of each spell element, or by a more complex formula.  (this would be similar to how money costs of custom units are determined in various games.)

The same principles would apply to other elements, so, for example, life might start with healing and defense increasing abilities, and go from there, earth might start with defense increasing and slowing and go from there, etc.  In elemental, spells using different elemental nodes could be created by combining spell elements of the separate magic elements (Hopefully it is clear what I'm saying.  )  So, for example, a volcano might be created using earth's "raise terrain", fire's "AoE direct damage", and overland abilities from either of them.

(note:  Examples are not meant ot be taken as actual abilities to be associated with each elements if such a system were implemented, they simply were conventional examples ot keep things simpler to understand.)

The main issues I can see for such a system are that it would be more difficult to balance (for the same reasons that custom units often are), and that there could easily be a lot of overlap between different magic types (since there aren't that many different individual spell effects, and separate magic types would likely overlap in different elements of spells, leading ot overlap in the spells themselves.)  There are likely a number of kinks that would keep this from actually being used in the game, but hopefully it is at least interesting ot read.