Yes, good one, I was thinking steampunk, too!
That would be one example of modern-retro in an alternate future. If it were in the present time, though, it's a bit too eccentric, but it's a good example.
Another one on the opposite end would be, say, the White House which has all of the modern amenities but it completely looks timeless in design with its Neo-Classical Greco style (Art "Greco"?)?
But, yeah, as you point out, it's hard to figure out a term. Maybe we make one up like "retronaut" (= pioneering or re-imagining the past designs with a modern twist / materials while staying true to the past nature of the design so as to maintain its nostalgic appeal - which is why one would be pioneering like an astronaut instead of simply reflecting on the past.