Let us suppose we manage to mine water on the Moon and return lots of it to LEO. Would it be hard to design an upper stage that could refuel at that depot, and uses that fuel to return to the surface and be reused?
I was reading about ULA's Advanced Cryogenic Evolved Stage, and in a Defense News article Tony Bruno said
"We had the idea, well, why do you have to bring it back to Earth just to reuse it?" Bruno said "Why don't we just leave it in space?"
...Yeah, but... why not bring it back to Earth and reuse it?
Is it just a matter of how to have enough fuel up there so that its reasonable to propose refueling a bunch of specialized upper stages with enough fuel to propulsively decelerate and land? Are there other complications?
Maybe I should mention that I'm thinking about this in terms of long-range lunar development. Jkavalik mentioned in comments that a big complication is that things are launched to many different orbits and so it would be rare for an upper stage to be in range of a fuel depot (though they mentioned an architecture change where payloads are transferred to space tugs that complete the orbit insertion process at the depot could get around that). In the particular case I'm thinking of, the payloads are destined for one staging area before continuing on to the Moon, probably a space station.