We want to enable extensions to provide custom settings UI to extend the (more and more) minimal settings UI that is build into Chrome.
Could this API be part of the web platform?
Do you expect this API to be fairly stable?
What UI does this API expose?
An extension using this API can display arbitrary HTML as part of Chrome's settings UI.
How could this API be abused?
An extension using this API can pretend to e.g. manage your passwords, but instead send all passwords you enter to a third-party site. A possible way to mitigate this risk would be to disallow all communication with other extension pages or network resources from settings page. An extension could also mess up your settings, but it already can do so from the background page.
In order to avoid an extension accessing the APIs available to Chrome's settings UI, the extension settings page would be rendered in an sandboxed iframe.
How would you implement your desired features if this API didn't exist?
An extension could define a browser action or an options page and allow the user to configure settings from there. Option pages, however, are meant for settings for the extension, not settings for the browser. Browser actions aren't where the user expects browser settings either.
Are you willing and able to develop and maintain this API?