After completing my first game programming tutorial for XNAinfo.com, I've realized something:  I like writing these things!  So I've been spending some idle time musing about future subjects for which a new article or tutorial would be of use to the XNA community.  Here's a few possibilities I've come up with so far:

• Debugging shaders and profiling with PIX.  I can't tell you how many times I've replied to a thread in the gamedev.net forums by saying "just step through your shader in PIX!".  PIX is undoubtedly a huguely useful tool (even when it's occasionally crashing), and I think it's important that beginners get comfortable with it early on.  There's documentation in the SDK, but I think a lot of beginner XNA programmers don't think to look through there and therefore don't understand how powerful PIX can be. 
  
  
• HDR and tonemapping.  It's probably pretty advanced for your average XNA project, but thesedays everybody wants to know how to do it.  There's some great samples in the SDK but once again I'm not sure all XNA programmers want to go poking around in there, especially if they don't know C++.  Plus there's always cool things I could throw in, like the LogLuv encoding scheme Rim and I worked out.
  
  
• Skyboxes.  Another topic I see brought up all the time on the gamedev forums.  It's a very very simple technique once you understand how it works, but unfortunately I don't know of any tutorials devoted to that topic.  Plus there's always a few advanced things I could throw in there...like RGBE (or LogLuv) encoding for HDR, or comparing DXT compression vs. uncompressed.
  
  
• An article or maybe even a series about map editors.  I've been working on a map editor for my own project, and I'm constantly amazed at all the cool stuff you can do with .NET when your engine is made with managed code.
  
  
• A follow-up article for my ModTool tutorial that discusses instancing.  There's an excellent Instancing sample on the creator's club website, however integregating the the technique with the content authoring system I described can be tricky (it requires modifying the shaders quite a bit, and also requires making some changes to the content processor).