To summarize this thread so far::
(1) we seem, so far, to have concluded that we may be approaching the limits in terms of "compactization" and weight reduction, in the very-short-tube Cassegrain/Ritchey-Chretien/CDK type designs, and in the short-tube (say F3.5) Newtonians of a "modified or improved Dobson" design, especially if the design also incorporates the use of advanced/expensive/light-weight materials.
(2) However, the idea of a Folded Newtonian seems worth developing
(as per a previous post in this thread), at least for the very biggest instruments, so as to bring the focus down to a manageable height.
(though the central obstruction is huge in some of these instruments, these are optimized "Light Buckets" for visual Deep Sky work, rather than planetary viewing, so maybe this can be tolerated)
http://www.siderealtechnology.com/28inch/
(3) Also, the ongoing Design Trend of reducing the weight of the primary mirror, seems worth continuing with. Let us hope for further innovation in this area.
(the 25mm thick 20 inch Primary Mirror in the previous post only weighs 10(!!) kilograms......impressive!)
As I've said, we're not talking about scopes for planetary viewing here, or about achieving the sort of optical perfection that the purists want!
We just want "a whole lot more light" for Deep Sky viewing, and to reduce the size and weight of our Gigantic Instrument to the absolute minimum.