This is exactly what it sounds like. A skill that allows building spells (like kata forms, but magical) and use them in combat. The way you would create one would be something like...well, uh...lets see here. Something like SPELL CREATE <name> <element> <targetmethod> <type> <typeargs>
the element would not really be needed if it would be given to all the mages, so you can just get rid of that...the target options would be something like this: spellcaster only, one target player, one target distance (up to 5 rooms away or something), all allies in the room, all enemies in the room, and a distance variant (only a couple rooms away though) for the all allies and all enemies type. The type, well...uhh.... would be something like...different kinds of projectiles (like needles or magic flying spears), different types of illusions, different kinds of defensive magic, basically allowing you to customize pre-existing spells make your own spells . The typeargs would support to type by being a bit more specific.
So. Just a vague idea, nothing much. The mana cost would depend on what you create (duh). I'm kinda wondering if the mystical and magical Ianir will consider putting this in, because it would be pretty cool to see.