An XNA tutorial I've been going through for 2D gaming uses a simplified version of Scorched Earth to introduce a lot of concepts like Texture, turns, angles, drawing, tradeoffs, etc..
http://www.riemers.net/eng/Tutorials/XNA/Csharp/series2d.php
There are a number of other tutorials (for XNA), that if you are brave, might give you some basic game ideas to translate into the particular environment you will use.
An alternative strategy though is to start with a game you love and try to make the next version of it. An awesome XNA game (incomplete) is a sequel to a Microprose game called Birth of the Federation. The author used it as an excuse to learn all the latest and greatest MS technologies. http://www.codeplex.com/Supremacy