I work in a laser lab, and we use off-axis parabolic mirrors. Essentially, these are parabolic mirrors, but instead of the incoming light and focused light going more or less along the same path, the ones we use are offset by 90 degrees. So, in a top-down view, our laser comes in from the left, hits this off-axis parabolic mirror, and is focused to a plasma below. Naturally, I wondered about using this for a telescope. The largest off-axis parabolic mirrors I could find are from edmund optics, a company that makes lots of optics for spectroscopy and laser systems. You can peruse and find the 4" ones here: https://www.edmundop...Mirrors/39488/
Essentially, what I'm envisioning is a tube or shroud in one direction to shield the mirror from stray lights and/or dew, and another tube to the focuser. No other mirrors or diagonals necessary. An attachable focuser at the sky end could hold a laser collimator, and an attachment of circular rings centered in the normal focuser would help you achieve collimation. Since most of the mirrors listed here are very short focal length (f/1 or f/1.5 for the 90 degree off-axis parabolic mirrors), you could get a huge area of the sky if you can correct for the coma - perhaps by stacking Paracorrs? Some of the other angles they offer are longer focal lengths, so could require just one Paracorr, or none if you find it's not too bad of coma.
One of the advantages would be no more silly arguments about whether a central obstruction is limiting your reflector. And you still get the advantage of a mirror: you don't have to worry about chromatic aberration. Of course, a large disadvantage is how difficult these mirrors are to produce. There's a reason a 4" one is the largest offered. There's a quoted surface roughness of 50-100 Å (turned on my Norwegian keyboard for that character, even though that measurement is named for a Swede), which is very good (and needs to be for laser applications). But I have no idea how one would do standard tests to determine the quality of the mirror without just trying it out.
Any thoughts? Concerns I haven't addressed? I haven't thought about the mount at all, and I won't, but feel free to suggest good ways to mount such a design.