Spell Research Interface Idea
Mixing for Custom Spells
Imagine a picture of a bubbling Cauldron. Around the main Cauldron are a bunch of smaller Cauldrons that correspond to the various elements. The player would use a pointer that looks like a hand holding a magic wand (think old school MoM pointer). You hold the pointer over a container that has a Element in it, like Fire, or Earth, or Air. You click and hold down the mouse button and suck up magical power from the small container. You get a percentage reading of how much "Magic" of the Element you pull out. You then hover the pointer/wand over the Big Cauldron in the center of the screen. When you click the mouse button again you "dump" the magic you're holding in the wand that you just took from a container into the Main Cauldron. The player is free to mix amounts of various Elements to try to make Custom Magic Spells.
Lets say the player mixes 50% from the Fire Element and 50% from the Air Element. They then advance turns while the ingredients boil simulating research time. When the spell is done researching a scroll can float to the surface of the Cauldron or the words for the spell can appear on the surface of the liquid in the Cauldron and the player is notified that their researched produced a new spell. The end result would be a Fire Tornado or whatever the Devs decide mixing that combination would produce.

No matter what mechanic you decide to use for the actual Magic Spell Research this type of interactive interface can provide fun for the player and give them the feeling they are actually "mixing" custom spells (could be used to make potions too). This also minimizes the feeling that the player would be micromanaging spell research because they would have a active part in it.