I have been thinking one idea for better utilizing the HALE concept (HALE = high altitude, long endurance).
Usually nobody flies higher than about 40000 ft. If you look out from a commercial passenger aircraft's window, what you see is blue. You can don't even see clouds very well since you are too high to see them closely and you are too low to see the curvature of the Earth and blackness of space. And the publicly available photography from that altitude is very limited, you don't really get to see even virtually how it looks like up there.
There are some interesting videos about balloon flights to high altitude in Youtube. The balloons go to about 80-100 kilofeets. According to videos, that looks already almost like space. Couple of examples:
Long Trail School High Altitude Balloon at Youtube
Nevada BalloonSat at Youtube
The view is so amazing that I feel it odd that nobody has started to carry people to near space experience with high altitude aircraft. Someone offers MIG-flights, but that is just a ballistic jump there from supersonic flight. Aircraft that can loiter in that altitude would give a whole different experience, it could stay there longer than just minutes.
That kind of aircraft would be impossible someone might say? Not so black and white. There are couple of HALE UAVs around which can go this high. And if you for example look Scaled Composites Proteus which can reach 70000 ft, if you'd replace the telecommunications load from the center section with space grade pressurized passenger cabin, the plane could lift several people at one time to the abovementioned altitude.
According to material I have been seeing from high altitude balloons, it seems like the sky is starting to look like space from about 60000 ft upwards. You need afterburning jet engines to go that high? Not necessarily. Look at for example Burt Rutan's UAV that had twin turbocharged modified Rotax 914 (with fuel injection). It was designed to have positive climb rate at 63000 ft. Seems feasible with piston engines in other words. The company that did the Rotax-conversion for the Scaled Composites UAV, have done triple turbocharged and twin turbocharged versions of the Rotax. The triple turbocharged Rotax is usable to over 80 kft, however, the installation looks really complicated (and the biggest turbo is so huge that must be from a truck).