Integrate PulseAudio with Xming/XLaunch (fun, but a distraction, and I need to check all dependent libraries are compilable with MinGW).
Fix a 'borders black, corrupted or missing' rendering error in -mwextwm mode e.g. with xcalc and oclock (tricky). XQuartz probably has the same miext/rootless error, and an API change (13/9/07 'Remove the PaintWindow optimization') seems to be the cause.
Fix a 'paste from X client to parent PuTTY console' error with -multiwindow (tricky PuTTY error, code unknown). The X clipboard selection from a child client is not available for GetClipboardData() in get_clip().
Complete the transition to modular builds (messy and time consuming).
Use the Mesa project from the git repository. Build its demo X clients for Windows and incorporate 'latest code' dlls into the Xming Mesa server (time consuming).
Update to gcc version 4. MinGW-gcc has been lapsing since Danny Smith left the MinGW project (could be a show stopper in the long term, but Danny is still active on the MinGW mailing lists).
Get -pseudocolor to work with the various engines (tricky, I need a test 8-bit colour *nix system to progress this).
Re-work the engines to suit latest modular shadow and mieq code (messy and time consuming). XQuartz has the same task to complete.
Continue the hunt for thread problems with the -clipboard code (elusive).
Use RandR (resize and rotate) to enable dynamic display size changes (should be easy, but probably isn't!).
Build a dll version of libXfont with 'weak' local stub function resolving that allows external overloading (you will then get a slightly smaller installer). The linkage has always been static when building Xfont into Xming and Cygwin/X because of this stubs nonsense. By using a 'trick' the dll may be made usable, see libXt/src/Initialize.c for something similar, and SharedLibFont for the background. Also I believe that __attribute__((weak)) is not supported by MinGW-gcc version 3 but is in 4 (not yet fully released)