Game extension is an entertaining activity that offers an opportunity to test new design approaches by non-programmers. The real challenge is to enable this activity by means of a suitable infrastructure. We propose a knowledge-driven approach with natural game-player concepts. These concepts, found in game ontologies, include game abstractions and rules for game moves. The approach has been implemented and tested for board games. These include tic-tac-toe as a simplest example, enabling extensions of tic-tac- toe, say to a four-by-four board and Sudoku, a single player game of a very different nature.