It's likely we will see a return to hydrogen peroxide as a propellant. Creating it from water ice is possible with vacuum distillation. It's not quite as effective per kilo than other propellants, but it's one of the easier ones to create in-situ. You just need to import silver from Earth (perhaps other catalysts exist too). I can imagine deep space vessels having their own small hydrogen peroxide refinery onboard, and carrying a giant chunk of ice. No need to wrap it all in metals, just attach the spacecraft to a large ice block and let it convert the ice to fuel as needed.
Regarding metal structures, vacuum-deposition techniques lend themselves well to creating lumpy, sphericalish pressure vessels, where a custom balloon would be inflated and then coated in a mist of metal particles to build up the form. Using this technique in vacuum, zero-g environments isn't very well researched afaik so lots of speculation on what may emerge. I doubt there is any need for aerodynamic shapes, and even cylinders are more difficult and less efficient to construct than inflated spheres and blobs. See https://worldbuilding.stackexchange.com/questions/180460/would-inflating-hot-spheres-of-metal-be-a-viable-zero-g-vacuum-ship-building-te for some more discussion on forging and construction in space. It's not impossible to construct flat sheet metal and build craft out of it, but with a suitable mold you might possibly just deposit the metal right into the final form, skipping sheet metal and welding entirely.
There is little need for exact symmetry. If you put a gimballed engine under, and stick RCS thrusters around, it's quite possible to control any odd-shaped spacecraft. The symmetry you see in todays spacecraft is entirely due to aerodynamics, manufacturing processes, and structural strength. Little to none of this applies for spaceship created in space, so it's quite likely you'll see bulbous pressurized compartments, with a bunch of stuff stuck on, seemingly randomly, all around the outside. The deeper into space you go the colder it gets, so expect a good degree of thermal blankets wrapping the ship.
It's not likely that ships will experience intense g-forces (unless maybe going to and from the surface of Mars), so spindly, thin structures will be possible. Radiators, antennae, delicate instrumentation and solar panels could stick out at all kinds of angles, although use of solar panels isn't super practical at the asteroid belt and further, at least with todays efficiencies.