Just in poking around a found a few interesting theoretical ideas.
One that stood out to me is actually to use liquid mercury combined with centrifugal force that causes a parabolic shape to be formed, making it ideal for telescope (I'm unclear how this would work in zero g, perhaps microgravity is enough). These have been built on the ground, but have some pretty big drawbacks here on earth, the first being that it can only sit vertical, the second being that it's extremely sensitive to any changing conditions. An immediate draw back would be potential containment, if you have in stop spinning for any reason, the mercury would just float off (far from ideal). I'm sure someone smarter than myself could think of something.
The most pragmatic (with our current skill set), and Webb works with this system is using the hexagonal mounting system, making lots of small mirrors, each on servos that could be finely tuned as needed. These mirrors are extraordinarily expense, but perhaps with mass production you would capture some level of economies of scale. If you could combine that with 3D printing the rig in space, it would seemingly be expandable as you go. Obviously you would need either a human presence or robotic one that accomplish the necessary tasks of assembly and any troubleshooting as you go. With this idea, unlike Webb, which had to fold up in the most complex origami man has ever created, the unfold and have everything go exactly perfectly (what an extraordinary accomplishment), you'd have some leeway.
A solid mirror, ground and polished, is still the ideal scenario, but for now, making that a reality on a large enough scale in terms of size and in terms of replication seems well beyond current abilities.
Those are just a few ideas that popped up from a google search, I'm sure there are a lot of other ideas with a lot of promise out there.