After the death of my last hard drive, I turned to Google Docs as my primary writing app, but drawbacks occur whether you save in chapters or save as a complete novel. A master document tray would solve my primary gripe. Sigil can easily separate one HTML document into many, and it basically uses a master document tray.
Sigil is open source, Google should be willing to work with Sigil to add features. In Sigil, HTML documents are lined up in the side panel and you can easily divide HTML docs. Sigil uses Header functions to control what goes into your Table of Contents; which, I find better than the Google Docs feature of outlining.
A Google Docs and Sigil combo, along with the better grammar checking would place pressure on Microsoft. The simplicity of Googe Docs is great, but it can’t match the power of Microsoft Word. I’ve got old versions of Word, but I get by with open source stuff, now. Many sites recommend LibreOffice, but Apache OpenOffice has worked better for me and for some stuff I need these open source apps because Goggle Docs lags in a few areas.