The roadmap can be viewed on the Jira project dashboard.
Discussion for Fabric releases happens on the #fabric-release channel in RocketChat.
Beginning with the 1.2.0 release, the Fabric maintainers plan on publishing quarterly, focused releases that target a small number of themes for improvements and new features.
We maintain a JIRA release dashboard for the upcoming release. The top left gadget on the dashboard indicates which Epics are currently in progress for the upcoming release.