Variant: Black Hole Go

[isaiahstjohn]

You may place your stone on top of a stone of the same color. Once N stones are stacked at a given location, the stones collapse into a black hole. A black hole is like an empty space in that it creates a liberty for vertically and horizontally adjacent stones of any color. But unlike an empty space, once a black hole is created somewhere, a stone can never be placed at that location again.

[JaniM]

And what do stacked stones do?

[isaiahstjohn]

They would be treated like one stone for all intents and purposes until the threshhold is reached that triggers a black hole.