This makes me imagine a Lord of the Rings-style aesthetics for the setting, with all the rural-medieval type folks living their lives in the shadow of the ruins of a glorious past. And sweeping vistas. Lots of sweeping vistas. Or something like Split in Croatia, with the medieval houses built into the ruins of Diocletian's palace (and, of course, all the repurposed ancient buildings used to this day in Rome).
That would work or maybe bump it forward a little so you have things like trains and steam engine boats and maybe limited projectile guns and riffles. The other way is something more like "Battlestar Galatica" were they are really us present day people but with a few nudges here and their that keeps it being a exact copy of our society. Like maybe they can travel in space but only have warp 2 and even if they could leave their system they are afraid to because the have heard scary stories about what is out their in the deepest reaches of Space. The Borg alone would make for a good futuristic Boogey Man even if nobody even really knows what a Borg even is.
Jason