What would be a nice feature would be able to jump into the circle without fear of losing/reducing the level of your arcane focus. Notice soon as a level 6 Arcance Focus is acheived everyone runs like a bat out of hell out of the circle because they don't want to be caught in there alone and have someone else recast Arcane Circle.
Perhaps too over complicated to code but I think losing a higher level circle detracts and prevents players from helping other players achieve their focus.
It would be a nice feature.