There are many ideas for building extremely large telescopes, and a number of groups around the world are currently working on design concepts. The world of astronomy is fortunate to have available at the present time a number of experienced scientists and engineers who have recently worked on the current generation of 6-10 meter telescopes. In recent years, many of these technical people have turned their attention to ELTs.
A number of different design concepts are described in the web pages listed below. Some of these served as initial concepts leading to one of the three active ELT Projects. Because these are largely of historical interest, GSMTPO cannot guarantee that all links will remain active.
See http://astro.nineplanets.org/bigeyes.html for a developing list of The World’s Largest Optical Telescopes (Operational, Under Construction, ELT Studies, Other Interesting Projects). This is a non-NOAO website, so NOAO does not guarantee accuracy.