Designing and building a new space telescope takes such a long time that even before Hubble 2.0 (the James Webb Space Telescope) has launched, astronomers are already working on Hubble 3.0, known right now as ATLAST.
The Advanced Technology Large-Aperture Space Telescope will be able to characterize the atmospheres and surfaces of habitable exoplanets within 200 light years. As its name implies, ATLAST (it's an intentional pun that will likely be changed long before launch) will be packed with Advanced Technology and one very Large Aperture. "Aperture" refers to the amount of light-gathering capacity that a telescope has; in other words, it's the size of the mirror that you can stuff into it. The bigger the mirror, the dimmer the objects you can spot.
The James Webb Space Telescope will have a mirror 6.5 meters in diameter. The largest optical telescope on Earth right now, the Large Binocular Telescope, has an effective aperture of 12 meters. ATLAST is shooting for a 16-meter aperture. With a mirror (actually a bunch of mirrors) this size, ATLAST will have an angular resolution that is five to 10 times better than the James Webb, along with and a sensitivity limit that is up to 2,000 times better than Hubble.
While bigger is of course nearly always better, there's a specific reason why ATLAST needs a massive mirror: it's designed from the ground up to be able to not just search for potentially habitable exoplanets, but to actually make measurements of their atmospheres and surfaces. Essentially, this thing is going to be looking for life on alien planets directly. With a 16-meter mirror, ATLAST will be able to analyze exoplanets within about 200 light years, searching for the spectral signatures of life. And if it finds something, that quite possibly would be the most absolutely amazing discovery, you know, ever.
ATLAST will (hopefully) launch sometime between 2025 and 2035, and since (unlike James Webb) it'll be put into an orbit relatively close to Earth, regular servicing missions should keep it running for at least 20 years.