At the moment, I suggest common chords in the same key as the rest of the song. I'm working on a recommendation system that will take more into account the music theory of progressions as well as common existing patterns found in different styles of music. Super excited to have a decent system for that, as it's one of the main things that I personally want to use in my songwriting.
Just a heads up, while having a basic chord progression suggestion tool isn't a bad thing, there are already VST tools that do this and integrate with a DAW, so you can hear the different progressions and export midi to a track which can then be used with any instrument. They also support various timings, different modes, inversions, etc. Probably not a good idea to spend too much time on this.
Right now it's closed, but I plan to open source at least some of the cooler pieces of it :) Likely some of the recommendation and audio stuff. Haven't had much time to think about that yet though!