Lack of such hang glider is #1 reason preventing willing people from hang gliding. The popularity of paragliding is proof of that (a paraglider packs into a big backpack). You can't store a hang glider in an apartment because normally it packs into a 5 meter long sausage bag. And I can't even go travel with my hang glider because where do I leave it when I sleep in the hotel? Too risky to leave it on top of car. A small package also allows flying at some ski resorts and other perks.
Designing this hang glider would be primarily solving a configuration problem – how to arrange all required components so that they are foldable and yet strong enough. You have 40 years of HG designs to get inspired from, and don't forget Princeton sailwing, that's an old, under-recognized concept imho.
By the way, hang gliders are small enough that you can truck-test them on the ground to make sure they're strong enough and have proper stability attributes. Unmanned flight testing should also be possible, though not done currently. HGMA even has a list of certification criteria online that you can use as a baseline. What I'm saying is that designing hang glider is easier than designing a Boeing. You do need to know what you're doing, but you don't need a team of PhDs in fluid dynamics and materials science. All important hang glider components are free to use, not covered by current patents.
I have no experience with hang gliders, but as long as you have rigid skeleton you have problems with trasport and storage. Could this skeleton be replaced with an inflatable one?
It won't work on paragliders because as ram-air wings, they need to collapse when they get unloaded. Otherwise once unloaded the wing will fall over in front of the pilot and probably get entangled with the pilot.
The only heavier-than-air inflatable aircraft currently in production is the Woopy – http://fly.woopyjump.com/ – its design is very unconventional, with continuously running pumps and some weird flight control mechanism. It never passed certification because it's probably not as strong as other hang gliders.
Some potential problems of inflatable hang gliders:
* Ambient air pressure differences at varying altitudes, temperatures, etc.
* Bird attack – sometimes eagles and other birds can get aggressive and try to claw a hang glider. What is a minor annoyance now could be a dangerous flaw if your glider is inflatable.
* A failed inflatable wing can potentially gift-wrap around the pilot preventing them from throwing the reserve parachute in time.
* Less rigidity than metal tubes – danger of buckling at higher speeds / wing-loading.
* There's a certain stigma among HG pilots regarding the concept of inflatable aircraft due to controversy around Woopy and the general idea of riskiness.
However, inflatable aircraft IS a practical strategy. The shortcomings listed above can be overcome given enough effort.
For example, see http://www.prospective-concepts.ch/html/site_en.htm – click on Projects / Air / Pneuwing (and other projects).
Also, http://en.wikipedia.org/wiki/Goodyear_Inflatoplane
Lastly, in all likeliness the only advantage of an inflatable hang glider would be portability. I don't think it will end up lighter than a tube+fabric construction. The guys from Prospective Concepts seem to believe that they will achieve better glide performance with an inflatable design, but to be honest I doubt that. But then again, they know much more about their technology than I do.
About the paragliders: you mean when they land? It is true that the landing becomes more challenging, especially in case of wind (kitesurfers often help each other when entering / exiting). But I guess this could be solved - if nothing else, only the central area could be "reinforced". Let's wait a few years, I'm sure someone will try it. :)
Normally a ram-air paraglider wing would collapse at low angle of attack due to lack of pressure inside the wing (it only works in a certain range of angles of attack), causing both the wing and the pilot to start falling, picking up speed and restoring angle of attack, and re-inflating again. It's kinda hard to explain concisely in more detail.
With a rigid inflated paraglider wing however, if the angle of attack becomes too low, the wing might get unloaded (falls faster than the pilot) and will pitch down even further because that's what a plank wing does in the absence of pendulum weight provided by the pilot. So the wing dives down and gets entangled with either the pilot, the lines or both.
I'm a bit rusty on the exact aerodynamics of paragliders because I only fly hang gliders, but this is roughly what happens.
If you want to know more about how ram-air paraglider wings stay inflated, check out this doc – http://downloads.flyozone.com/pdf/PG/ozone_shark_nose_en.pdf – it talks about a new ram-air intake technology and explains how ram air wings work.
Why I am doing this at 5 AM..? I'm insane.