About "fieldsep_csv", I do not like much the principle of having different output variables to represent the same concept depending on the format. I would rather have reused fieldsep as in your previous submission and set it to "," when under --csv.
yes
how will be possible to set different separator ';'? I don't see it with described design
Indeed, it should be possible. I think that the following should be made to work:
psql --csv -P fieldsep=; -c 'TABLE foo' > foo.csv
So that it can be changed the semi-colon (or tab or whatever) style if required.