I agree with Marc that it's simplest if each SRFI presents a
well-defined interface with as few options as possible; this includes
syntax.

If the community discovers after the fact that an existing SRFI's scope
is too broad, it seems it would be best to make a new SRFI (with a new
SRFI number) that is simply a subset of the old SRFI. This has been done
in the past: all the SRFIs with "reduced" in their title. Scheme
implementations that ship the old SRFI could also ship the new one, with
the identifiers simply being aliases to the old library.