It would be nice if it were possible to have separate permissions for the wiki and for the source code.
Such as giving to a user r/w in the wiki and either ro or nothing for the source code, you get the idea...
Use case: giving to non-programmers people involved in a project the ability to view or edit the wiki or use the issue tracker, without having any risk of them messing with the sources for some reason.
I don't know if it could be useful to have the opposite too (i.e. wiki ro and source r/w).