The work on the hyperchains whitepaper has been started and shall be developed on github. At this point it consists of some stream of consciousness and basic skeleton of the document that is to be iteratively polished and upgraded. Note that it is private at this point and you need to be in aeternity organisation in order to have access to it.
Beside that, there is some overall plan and risk analysis done by @gorbak25 here. Ideally we should merge all the information from this work into that repository. Also, would be nice to have GH pipeline to build that PDF (latex). Everybody is welcome to contribute, give feedback and point out any doubts or mistakes.