Some suggestions and features based on personal preference, so I'm putting in a separate comment.
- Put an "Import MIDI file" button in the Imported Files section
- Star or bookmark icon, pick one for favorites. Currently one is used on the library screen and a different one in the song view.
- Sidebar for the different sections with hide/show toggle, for example I don't like to scroll through stacked Sections and don't really care to see Recents at all
- Options for thumbnail, tab display could be one but some variations would be interesting:
- Color based on tempo
- Dot-based graphic (primarily aesthetic, but recognizable at a glance)
- Size S/M/L option which assigns per section
- Tags for tracks, ideally color-codable. Maybe I want to tag something "mastered", or "difficult", "somber", etc.
- After entering the search box, escape key or outside click should dismiss and return to the home layout. Currently the sections get reorganized, Favorites and Recents disappear and Pieces shows up at the bottom, but I can't get back without closing the app.
- "Choose Random" button to open a piece
- Composer interface, create new pieces from scratch or modify from the library using something similar to the playing interface
(n.b. a decent interface for this if no keyboard/midi-in would be present a 1+-octave keyboard with some modifiers, so multiple key taps can create individual notes, optional rest interval, or extend a continuous note. Row-level actions so you can duplicate creating repeats, shift up/down, etc. Slight differences between macOS/iOS.)
- Allow user editable text, emoji, or color highlight in the play scroll. Noting a section of the piece is one obvious use but it could be handy for several uses.
- (semi serious) watch integration to vibrate on tempo.
- Music box mode, notes only play as you scroll (I see you did this actually. My daughter is gonna love this...)