One I thing that would be quite useful and would even solve
Gabor's problem:
The introduction of a new command line switch, say "--build-ignore",
to the commands 'R CMD check' and 'R CMD install'
Shouldn't that option be "--ignore-build-ignore"? :-)
More seriously: I suspect that an on/off switch for the .Rbuildignore
file wouldn't be sufficiently flexible: the same package author might
want different things at different times, e.g. building a binary for
Windows would include different files than a binary for another
platform. We could put together some sort of system of conditionals
within the .Rbuildignore file, but I think it would be better to just
advise such package writers to maintain one or more .Rbuildignore.in
files, and manually (or by makefile) copy the appropriate one in place
first.