ordinary users may have to change to a system
Since you ask for comments ...
My expectation (as I've expressed more than once to Frank, Will, and
others, by email and in person) is that any such latex3 format will be
invoked as "latex3", and "latex" and "pdflatex" will remain entirely
unchanged, invoking latex2e. Thus no users would be affected unless
they choose to be.
Otherwise, as far as I can see, essentially the entire LaTeX-using world
will be broken, and that will harm everyone.