To be fair, creating a new game that's good takes a lot more talent across a lot more domains of expertise than your average FOSS hacker might appreciate. The code is just one part of it - you've got art, music, level design, story possibly, it can be as complicated as making a movie.

Cloning an existing game is easier.

If people want to try their hand at it, I recommend them to try to participate in game jam warm-ups, such as MiniLD[0] for Ludum Dare. Getting your feet wet gives quite some hint at the scope of things you have to handle.

[0]: http://ludumdare.com/compo/minild/

I recently discovered Argentum Age, a high-quality, fully open-source game that is certainly inspired by other games, but also feels quite novel in many ways. My understanding is that it has some of the same folks that have worked on Battle for Wesnoth, so I have every confidence that the team can execute on their vision.

http://argentumage.com/

Game design is a broad study and difficult to do well. Programming a game based on an existing design is the easy part.