As a VISUAL instrument, though, it would have its place. You can put a larger focuser on it and allow the use of 3"-eyepieces, for instance. A 30ES-100 would yield a staggering 3.3°-FOV with 6" of unobstructed aperture; an absolute dream for widefield junkies like myself.
Since this is for visual and Markus wants to make it *affordable*, I would avoid the 3" focuser. The diagonal and the eyepiece in question adds ~$1600. Of course, if a 3" focuser is at a minimal price difference, it does not harm.
It's not clear to me what are the constrain limits. "Cheaper serial production" and calling for ideas seems to be a bad combination, or a call to chop features.
My personal view on refractors for visual use is that those instruments are for easy setup (no collimation, quick thermal stabilization) and/or for wide fields. After that there are things (high magnification, light gathering ability) that other designs solve better at a similar or better price (but with other compromises, of course).
What I would ask for a refractor is a retractable dewshield and a good focuser able to handle a 31T5 with a 2" diagonal. The glass is decided, and it's a good one. I like "whatever matches best", it does not have to be lanthanum. You can't get much better than that, even if expensive fluorite is used. The cost of the optics are then only based on how much time one wants to invest in the journey to perfection.
Cost aside, my very bold idea is a tube that you can split in two parts fore easier transport. A bayonet like the one in camera and lenses is an example. The rear tube could be smaller to fit into the front part, with an adapter ring/flange in between. Imagine flying with 6" refractor in your carry-on cabin luggage!
I won't ask for a flat field telescope, this is not part of the "affordable" concept. However, a visual-oriented dedicated flatener would be a plus for those that dos not tolerate field curvature. If Jon says he does tolerate a 900 mm FL refractor, I take his word. I can tolerate my 600 mm FL refractor.
I certainly would not like having any kind of permanent blue light filtration. Such filtration can be added on demand with a 2" filter with negligible impact in performance, but if I'm in a dark site and want to see galaxies I also want the full spectrum to hit my eyes.