While I'd love to support importing every conceivable music file format there is, there are many practical limitations. For initial launch, I decided to pick 1 file format which met my requirements. That is MusicXML. In the table below, I've listed a dozen music file formats and marked whether they do or don't support specific requirements. Because all music notation apps, Finale, Sibelius, Dorico, MuseScore, Logic Pro X & others, can export to MusicXML, all professionally-written music content can be exported into MusicXML.
The biggest take away is that almost everyone sells .pdf files of 'sheet music'. Unfortunately, since .pdf's files do not actually contain music notation data, but merely contain pictures of music notation, they won't work for importing rhythm & pitch. Expensive and poorly-performing OMR 'Optical Music Recognition' attempt to read such files, and many of them export MusicXML, but almost always require some clean-up. Some proprietary file formats are also quite popular, but require the user to purchase an additional $$$ program to open (and they could be exported to MusicXML by the content publisher).