Some topics I am considering:
C++. Dirty secret: although I've mostly blogged about C# programming, much of XNA itself is written in C++, so I've actually been straddling both worlds for years. And interesting things are happening in C++ land right now, with the arrival of C++11 and maturity of the standard library affecting best practices and recommended style. But are my readers interested in such things? You tell me!
More about how GPU's work. Tiling, swizzling, caching, parallelism, predication, etc.
3D Math. Vectors, matrices, what really is an orientation, how to think in multiple coordinate spaces, and why rotation angles are Teh Evil.
Game design. What makes it fun? What makes a good AI? How does one persuade customers to try/buy your work? I've been reluctant to cover these topics because it's not really my area of expertise, but I do have some opinions about it so who knows, maybe a few worthwhile articles to be had here.
SIMD optimization, SSE, DirectXMath.