I agree that a Spanish translation would need a separate Expression record, with a qualifier of some sort. And a different Spanish translation (i.e. different translator) would require yet another Expression record, with a different qualifier. Difference in translation is clearly a difference in Expression.
But what if Harper Collins published a Spanish translation, and a year later MacMillan published the exact same translation, perhaps even with the exact same art. Would you give this yet another Expression record? Even if they are exactly the same except for publisher? Differences in cover art, publisher, and minor added material like introductions are considered to be part of Manifestations, not Expressions, according to FRBR. FRBR doesn’t ignore those differences, it just considers them to be in a different category.
I suggest that the differences you are talking about are differences in Manifestation, not Expression. We would not express different cover art in authority records for individual titles; that would be handled in bibliographic records, as information about the Manifestation, not the Expression. Just as a Manifestation of a single Work expresses the minor differences in cover art, introduction, and publisher, perhaps we should consider Manifestations of series Works/Expressions, which distinguish identical series differing only in these minor details. That would satisfy your desire to distinguish these things for the user. In a new RDA setting, I can imagine having Series broken into Works, Expressions, and Manifestations just as we do with individual titles.
That would be a significant change, but it is the logical extension of considering a Series as a form of aggregate Work, with Exressions.