For most issues and feature wishes, you can use the bug tracker.
You can also join the
#mingw-w64 channel on OFTC (or on freenode where messages will be relayed to OFTC).
The subscription is done over email and, after the first one, the mailing-list sends a message. It is possible that it gets marked as spam and you should therefore check there if you don't receive a confirmation within a few minutes of your subscription.