After Webb: Scientists Make the Case for Even Bigger Telescopes

With NASA’s James Webb Space Telescope set to launch in late 2018, researchers are already talking about the next generation of telescopes. That’s because massive instruments take years, if not decades, to plan, build and launch, and scientists don’t want the science to stop when Webb wraps up its mission.

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I imagine by the time the telescope (any space related instrument really) is up and running much of the technology is out-dated.
Its ability to withstand the rigours of space might also mean the equipment is robust but not quite state of the art as the built in redundancies don’t allow for this.

Following links from OP, the front runner mentioned is not only smaller – the same size as Hubble, but even older than Hubble. It’s a war surplus space telescope no longer needed for Earth observation.

The idea of using a separate star occulter needs extra thought as well, because moving it from one star to another would be a pain.