Most Important features
I applaud the Goodnotes developers for making Goodnotes available for Android.
But to be frank, the app is unusable right now, and the devs won't be able to gather enough data to considerably improve the application .
I suggest two main features to add immediately if you want to make it usable for the mean time.
Making a dedicated android application, and not some Web-application ported over from Windows. It makes no sense to improve and add features on this web application, if the goal in the future is to make a dedicated android app. Just make one now and improve on that base.
Make the S-pen compatible with Goodnotes, and I mean mainly to be able to use the side-button as an eraser.
If you can nail those two features, then we will have a usable app, from which we can improve on, and hopefully arrive at feature parity with IOS.