We have just started playing with Redmine in our office
and we found it to be very cool on of our Colleague said: "this is better than Facebook"
I think a good addition will be an addition of achievements and medals like for example
-5 bugs in one day
100 bugs resolved (200,500)
5 features implemented in 1 week and so one.
I now it is probably not appropriate for such a simple tool like Redmine but i think it will work for a plugin.