Redmine

As far as I know it is possible to customize the theme a little bit, css changes are possible etc.

What I miss is an easy to use possibility to add text like you have done it right here in the "new issue" page ("Before submitting a bug report, please read SubmittingBugs to make sure that you provide all the required information. If you just want to test Redmine, you can go to the online demo.)

What do you think about html-files, that can include user-defined html snippets, that are automatically included into the redmine html output if they exist in the themes folder.

For example a new folder could be used:
public/themes/THEMENAME/includes

that could include an
new.htm that is included automatically after the "new issue" headline
news.htm that is included for the news section
same for wiki, forum, project archive, activity view, download, and so on...

Every user would be able to include information into redmine without "hacking" redmine sources (and applying this changes after every update...).

I hope I could explain my idea to you...