More features like in the iPad version. Make it a native Windows app instead of it being a cloud-based app.
Currently, GoodNotes is web based and more like a cloud-based service.
This needs to change. It needs to become a native Windows app that runs natively on Windows instead through some hoops in order to make it run in a browser and then create some kind of GUI for it to make it look like a native app.
Add more features like on Apple iPad, inserting and exporting PDF's and pictures, make it cross-platform at least through the browser so people can access their notes on their phones and different devices. More colors like on the iPad.
As many others pointed out, just make it like the iPad version and release it when it is actually done. Beta stands for something that is near it's launch and not something that actually needs some work done to it like Cyberpunk 2077 at launch.
Otherwise, you guys are doing a great job and I hope to see all of the mentioned improvements soon on the Windows and Android version.