I'd like option for Chrome OS and/or Google Drive/OneDrive (Dropbox's too expensive) for iOS.

I mostly use Google Drive, I wish I could use iPad for Scrivener but I stick to other word processor like Pages or Microsoft when it comes to my iPad. I don’t want to have Google Drive and Dropbox at the same time… Also, I use Chromebook for web browsing and entertainment and my iMac for work. I would have to copy and paste text from non-Scrivener from iPad or Chromebook to iMac every time.

It’s covered elsewhere, but technical reasons why Google Drive is a non-starter for synching Scrivener, including very real likelihood of it screwing your project.

You can get Dropbox for free over 3 devices (more if you don’t install the app on iOS devices) and the free tier is plenty big enough for multiple books, plus easy to get additional free space.

No announced plans for Scrivener for Chromebook. There are plans for Android, but apparently some time in the future.

When (if) Scrivener Android is announced, apparently it will run on those Chromebooks that support Android on Chromebook, however the general Android app on Chromebook experience has been reported sub-par. Most Android apps don’t look much better than a Kludge on Android tablets, let alone the Chromebook, but who knows what wonders Keith and co will achieve.