Below is a link to an article that appeared in the Scholarly Kitchen blog this week:
The article suggests that the publishers of scholarly journals may create "mirror journals" in order to respond to the demand of the funders behind "Plan S" that their researchers stop publishing their articles in "hybrid" journals (ones that mix subscription-based and open access content). The mirror journals would separate the open access content and publish it as a separate series in parallel with the subscription-based content in the parent journal. Commentators on the article wondered how it would affect article indexing and DOI numbering. If it is adopted more widely, this pathway seems likely to add an additional element of chaos into scholarly publishing.