REQUIRED_USE supports only declaring 'hard' dependencies of two USE flags -- that is, 'foo? ( bar )' makes PM fail if USE='-bar foo' and require him to adjust the flags. This is mostly expected but there are cases when the enforcing flags are set in the profile.
For example, if a package that has an optional dep on Python stated:
REQUIRED_USE="!python? ( !python_targets_... )"
to express that PYTHON_TARGETS are meaningless without USE=python, PM would require user to unset all Python targets on the package which is not really helpful.
What I would see is a 'soft' counterpart to REQUIRED_USE. For example:
MEANINGFUL_USE="!python ( !python_targets_... )"
would solely 'gray out' PYTHON_TARGETS and avoid rebuilding the package on PYTHON_TARGETS change when USE=-python is used.
Runtime USE flags can serve as 'good enough' replacement.
I'm going to roll out a more complete proposal for this.