Our roadmap used to be contained in a single markdown file, but has since been transferred to a nifty Trello board where you can view requests, add comments, or even vote so we know what folks need.
To view the Trello board (that includes old requests that used to be in this file) please see the Office UI Fabric Core Requests board. If you'd like to make a new request, please file an issue in Fabric Core's issue tracker so we can add it to the Trello board.