Have a way by which people can add comments on specific parts of the content without affecting that content; and (more ambitiously) suggest changes that can be accepted very easily.

Commenting and suggesting modes are very well used in Google Docs, for the simple reason that they are very useful. In Wikipedia you have to actually change the text, and risk being reverted. Where there is a suggestion facility, it makes it clearer that the suggester is not certain whether it is an improvement in the eyes of others, and less face is lost if a suggestion is rejected than if an edit is reverted.

I have no idea how this is implemented, but it clearly isn’t simple. It could also make the data model quite a lot more complex.

Commenting is more widespread. Apart from Google Docs and similar, you could see Git as effectively serving the need for suggestion, but you don’t get the immediacy that you get with Google Docs. You could also see Fedwiki as meeting a very similar need, in that someone can make their own fork of a page, change some part of it, and the original author can bring the changed version back to theirs. But that is the only way you can do it on Fedwiki.