Trying to summon the devil seems so anticlimactic sometimes. I’d like to fix that problem by guaranteeing a success rate of 100%. What I mean is that every time you make a magic circle, any circle, you’ll be able to spawn a creature in it. Optimally, the type of creature should depend on the glyph drawn.
The main challenges of this concept will be the circle detection and glyph detection. I may have to compromise by making a set printout of the magic circle and then drawing the glyph, but other ways I can avoid that is to create a bounding box for the circle and having everything go from there, or printouts (which is last resort).
I may also want to add behavioral modifications to each monster: for example, the ghost can circle around you, or the devil can walk along with you and do your evil bidding.