Since I agreed with @Harroguk, I created a fork and started implementing searching for movies and downloading them. The first of my progress can be looked at here: https://github.com/galli-leo/Sonarr.
New movies can be added (even though they are just a series with one season and one episode in the background) and the rarbg indexer can be searched for movies. However, at the moment downloading fails. (So there's that.)
@fedoranimus It is very possible that you can import the database from Sonarr so that you do not have to setup everything twice. Actually, I think it's exactly the opposite. By forking Sonarr we have a lot of tools already layed out for us. I think the UI can stay pretty much the same.
And regardless, whether we keep the fork separate or not, we have to reimplement a lot of stuff already present.
Furthermore, as I have seen with my limited experience working on the fork, Sonarr is already pretty modular. Of course, changes to something like searching for a new series, we would not be able to sync. However, the rest (messaging system, indexers, etc.) should be no problem, if implemented right.
For example, someone just pushed a new messaging system to the main project and I should be able to merge that with my fork without a problem.
Additionally, I think a separate fork allows us more freedom. We could, for example, add more movie focused indexers (TorrentPotato) and change the way releases are searched for (predb). This could prove to be easier when working on a separate fork.
Lastly, after having worked with the code base a bit, I think it will be easier to gut the whole series stuff and build the movie stuff above it.