OK. Hear me out. I know that blimps float because of the buoyancy of air, and there is no air in space. But think of the way a boat floats on the top of water even though the material that the boat is made of is generally much heavier than water. The amount of water displaced is heavier than the boat and all of its contents, so the buoyancy is sufficient to hold the top of the boat all the way above the surface of the water.
So is it possible to float a huge blimp on top of the ionosphere? An ordinary blimp is filled with the lightest air possible - hydrogen or helium, which maintains the structure of the blimp with its air pressure. But could such a structure be made so large that it could float on top of the ionosphere and be filled with nothing, so that it doesn't even need a top to keep out stray ions? (Maybe I should say space boat rather than space blimp.) Is there any material that could be light enough while still maintaining a structure that wouldn't leak? How big would it have to be to displace enough ionosphere to float up there?
And to preempt the obvious question, "Why?", I'm thinking in the same general direction as a partial space elevator. The energy cost of getting to space seems to be a primary barrier to space travel in significant volume. And every little bridge that we can come up with to lower the energy cost might put space within reach for a few more of us little earthlings. The edge of the atmosphere is only a small percentage of the way toward the altitude of geosynchronous orbit and beyond, where a space elevator could ultimately fling stuff into space. But the atmosphere is also where all of the turbulence will hit the structure, so maybe a support structure at that altitude could provide some marginally significant strength to such a structure. Imagine a space blimp/boat thing in a shape sort of like a Bundt pan with support struts extending to the elevator shaft.