Hi Pieter

> Yes, it is an extension of the extramarks package. Regularly, I get
> questions (by email or on stackexchange) that can't be solved by the
> current implementation of extramarks because the marks are not
> independent. So I am experimenting with a new implementation based on
> the e-tex \newmarks mechanism.
> 
> I now use the package name extramarks2 and the macro names will have
> 'extramarks' as module part.

Depending on how closely it is going to be upward compatible (modulo bug 
fixes) it might make sense to consider keeping the name extramarks and 
offer a fallback to version 1, e.g.

\usepackage{extramarks}[=v1]

for people who really need the old version for some legacy document.

This has the advantage that one doesn't end up with several packages 
doing roughly the same and that people (who often just copy their 
preamble from one doc to the next) actually get the corrected/improved 
version by default.

How to make this happen is explained in

https://www.latex-project.org/publications/2018-FMi-TUB-tb122mitt-version-rollback.pdf


> It is actually fun experimenting, but I don't know if I will release
> the result. If the new marks mechanism becomes available in LaTeX2e,
> it might be easier to use that to implement it.

in that case, maybe extramarks-experimental (or extramarks2) is indeed 
better :-)

> In the meantime, I am also learning expl3 syntax, which is more of a
> challenge.

yes, we know that we are still missing a good introduction on top of a 
reference manual. A quite nice one is given at

https://www.latex-project.org/news/2020/11/09/l3-tutorial/

cheers
frank