I was using Sync before and it had a way to browse communities I wasn't subscribed to. It was nice way to discover new communities.
In Boost, if I tap search it shows me a small list of "trending" communities.
Browsing communities will be added for the next version. Currently you have the option to search for communities only.
Is there a feature roadmap somewhere that we can view or participate in? If not it might be helpful to setup a kanban board or track issues using gitlab or something, it would help me tailor my feedback a lot if I knew what was important.