You'd like to contribute to SonarQube but don't have a killer idea to start from. Here are several suggestions of possible plugins:
License analysis by integrating http://fossology.org/
Check spelling in Javadocs, string constants, ...
Integration of JTest from Parasoft - see SONARPLUGINS-99
Here is the list of plugins that were removed from the forge because they were deprecated or never released:
Source code of those plugins can be found at revision 6107 of Codehaus SVN repository: http://svn.codehaus.org/sonar-plugins/trunk/?p=6107.