I don't want to put limits on the end user (the game developer). Are you suggesting that the whole widget should best be a QGraphicsScene, and the user could hook in by either using signals and slots, or by subclassing?

Addendum: perhaps the grid itself is only one thing in the entire scenery. perhaps the scenery of the game he's developing requires more than one grid.

That's why I think the widget itself should be part of the scenery, not the scene itself.