My idea is simple. If you use a spell a number of times (i.e. 50 or 75 times) you would receive one spellement for it. This would reward players for grinding spells and reward the use of their favorite spell. The number of times needed to cast it would keep it from being too overpowered and it may inspire people to play more with spells that have been neglected because they know that will help make it more useful.
