These might be suitable for GSoC...

create a user interface that can be resized by grabbing corners...

• Better use of increasingly diverse screen sizes (Mixxx runs on netbooks with resolutions of 800x480 all the way up to Desktops with resolutions of 2560x1600).
• Reduce the number of duplicate skins (there are 5 variants of outline skin currently shipping with mixxx most copy the same artwork with different spacing or sizing).

• Render Mixxx on an OpenGL canvas similar to way it's done in the Qt4 demo: Qt Widgets enter the third dimension: WolfenQt.
• Try to apply Qt4 SVG support to buttons and controls.
• Investigate CSS stylesheets for helping make skin rendering more dynamic (i.e. using percentages for sizing and spacing).
• Have an option to lock aspect ratio, so skin does not become distorted.

Build an audio plug-in framework to make Mixxx's audio engine more modular and user extensible.

• Increased modularity
• 3rd parties could contribute input plug-ins by adhering to the proper api
• 3rd parties could also bundle codecs that they have obtained licenses for the benefit of their users (if a hardware vendor wanted to ship Mixxx for example)
• Users in locals where software patients do not apply can add plug-ins that can not be distributed in other locals.

• Refactor the existing soundsourceproxy (and soundsourceMP3, soundsourceM4A, etc...) to make it into similar self-contained input plug-ins that can be loaded at runtime similar to that implemented as part of the cmus project.