You can pass text among formats as long as you provide an unambiguous
mapping between equivalent entities and confine your markup to those
entities which are mapped.

In any case we will need to create a well-defined list of semantic entities
and attributes we expect PG to support. As long as those entities are
distinct the list can be as extensible as we want. But you are correct,
if there is any entity or attribute definition that isn't mapped, information
is lost. We can't  build a master format on any markup that doesn't
support the full set of definitions.

There's no existing standard for which we can agree to support everything
that particular standard supports. Even within a standard the same
semantic construction can be represented in various ways to various
degrees of detail.