So the main thing of interest in this post seems to be angular resolution. You're after objects that are relatively bright and small. Angular resolution increases linearly with aperture size. However, you don't necessarily need a giant light-bucket of a telescope to achieve high resolution. You can achieve some impressive resolution through use of interferometry. Interferometry is essentially about combining the light from multiple telescopes. The distance between the two telescopes gives a resolution that's about the same as if you used a single giant telescope with a diameter the same as the separation between the two telescopes. Obviously, as the light-collecting surface area is much lower, the two interferometric telescopes won't be able to pick up faint objects nearly as well as a single massive one.
The next 'flagship' optical space telescope is currently called ATLAST, and is planned for development and launch in the 2030's (which practically speaking, given the usual development cycle, means we probably won't see it till 2050 at the earliest, if it ever even makes it out of development). The design is still in the concept stage, and the primary mirror is slated to be somewhere between 8 and 16 metres in diameter (compare the 6.4 metre diameter JWST and the 2.4 metre Hubble). At it's largest, this would provide an angular resolution about 6.6x that of Hubble.
However, this resolution isn't nearly enough to resolve red giants with, let alone exoplanets.
The VLT, by virtue of the separation of its individual telescopes (about 130 m between 1 and 4 as measured by google maps because I couldn't find an official value), is able to achieve a resolution of 0.002 arc seconds, which is about 50x better than Hubble. We are now beginning to be able to see blurry images of the largest nearby red giants.
Picture of Antares, a red supergiant, obtained from VLT. Other imaged stars can be found here
So, what do we have to do to obtain an image of an exoplanet of similar quality?
The largest exoplanets are about ~10,000 times smaller than the largest stars. If you want similar resolution to the above picture, you need 10,000 times the effective diameter (separation) of the VLT. That's about 1,300 km, or about the 'width' of the Arabian peninsula from the Red Sea to the Persian Gulf.
I'm no astronomer, so I don't know the practical challenges of building something with that separation (whether in space or on the ground). I do know that the distance between the two telescopes must be known exactly, and not allowed to deviate. Putting two telescopes on (say) either side of the Great Rift Valley, would be a bad idea.
What about in space? Again, problems with the distance between the two telescopes will occur. The distance must remain exactly the same between them and the object that combines and records the light. The linked question in your question has an answer that gives some of the monumental difficulties with designing such a telescope.
In summary
In order to image exoplanets, you don't want a single big telescope, you want a set of smaller telescopes with a (very) wide separation between them, so you can do interferometry. But even for interferometry, the odds are stacked against you due to the engineering challenge of having a megametre-long but precisely measured distance between them.
