Gonna throw one of my own in here, I recently played an indie title, Mages of Mystralia which allows you to customize spells by adding runes that modify the spells behavior, and can modify elements in the late game.

It has essentially what is a board to build your spells where you have to place and chain the modifying runes by their connection points, which serves as the balancing, and part of the limiting factor of what can be combined and in what order.

Very interesting concept, but unfortunately the game is rather short, so you have very little time to experiment with the spells before the game ends, and while a sequel was announced, it seems the studio may have gone under during COVID.