first of all CONGRATULATION! This is the first project I found having exactly and almost all of the features I need, and mainly the multi-project features (wiki per projects, issues per projects and so on...). What is amazing is that i developed in Ruby!!!
Well I'm planning to use Redmine for my project (I'll give you more details if you want), and I would need two functionalities.
The first has been already requested the Feature #699 (OpenID functionality) and I really hope will be implemented.
The second one is a payment system, the idea is to provide the possibility to offer money for new features, bug fixing and support on project bases.
So imagine to have in this form a field where is possible to set a reward in money and the payment system.
I think this could become a "killer functionality", it could be useful to several projects.
In the hope you will be interested...