First of all, let me praise you for the amazing job you've done in creating this fantastic piece of software.
After having used the app for over a year almost weekly, I do have a good idea of a few of the shortcomings.
It's not really a shortcoming, but a feature that will imidately broaden your user base/clients. iOS version of the app for iPad.
80% of the musicians I interact with on a daily basis use an iPad with OnSong. Many of them are really frustated and willing to pay money to try something else.
Sync accross accounts. For example, let say you're playing in a band, and all the musicians have Chordle. You should be able to creat a set list and share it with them all. This way they can see the list and changes to the list and keys in real time.
Chord Degree Notation. Beeing able to, as a toggle, change from your preferred notation to a degree notation that shows the chords as degrees of the Key you're playing in, displayed as Roman numerals. For example, if your playing in C, C will be I (1), G will be V, F will be IV, Em will be iii (or IIIm).
The reasoning: When playing on a musical set up, often times you have to communicate chord changes with your musicians, but not everybody is playing on the same key, whether it is because they're using capo, transposing, using an instrument with a different tuning etc. Most experienced musicias will know this by heart, but some have never been exposed to this.
Another reason is that some songs require a key change mid song. And the current system struggles identifying keychanges.