Automatic beta updates
Currently, F-Droid supports automatic updates, but will bump the CV if it updates.
Beta updates, which would be downloaded and appear automatically, but not bump the CV, would be an useful feature, as it would allow keeping up to date with the latest beta releases without extra work. See Kontalk as an example of a project that pushes both stable and beta releases to GitHub.
I envision the difference between the stable and beta update path to be the regexp that is matched. If it matches the beta regexp, but not the stable path, it would be added without bumping CV. Of course, what is the best implementation is most likely better decided by someone else than me.