Ability to create major versions from within Excel Online (511 minor versions limitation)
There is no option to create a new major version while inside a Excel Online spreadsheet--you have to go out to the document library and publish a major version, then go back in. This is problematic when you are coauthoring a file and you reach the 511 minor versions limit--no one knows what happens next in a coauthoring scenario (does everyone have to get out and go back in?), and there is no documentation about this scenario other than the warning that the last version will simply be overwritten going forward (does everyone get the error message?).
Alternatively, you could remove the limit on minor versions.