As someone who's become used to composing with guitar tablature (a la Guitar Pro) instead of musical notation, it'd be great to see this mode supported as well. The fact that this has MIDI export built in is another great feature and a requirement to prevent lock-in. Good job.
That and the tooling is almost here with MIDI.js[1], Web Audio[2] and Web MIDI[3] as a working draft.
[1] https://github.com/mudcube/MIDI.js/
[2] https://developer.mozilla.org/en-US/docs/Web/API/Web_Audio_A...
In fact, latency and audio quality (because of file sizes for sampling and because of available resources for synthesis) are two big limitations in a browser.
We have big expectations about the upcoming Audio Workers ( https://github.com/WebAudio/web-audio-api/issues/113 ) to release advanced synthesizer.
Also, we want to enjoy the capabilities of the MIDI output features, in order to route MIDI signals to Virtual Instruments on the computer (through virtual MIDI ports)