I came across the blended wing body designs and their advantages etc. They are still very far from reality and require 20 more years till we see them fly. Isn't a conventional fuselage design with a somewhat flattened shape that generates lift during cruise only, a better alternative for now. Would it be possible? If yes, then that means the fuselage usually stalls in all other configurations but during a cruise at a particular AOA. That does mean that drag increases during climb and decent but the efficiency gained during cruise should mean that it is beneficial for a long-range flight?
I think you are looking at this backwards... (at least for a utilitarian passenger type aircraft)
First, such a design is most certainly possible, but it would create more problems than it would solve. Wings don't need extra help producing lift at cruise speeds, that is when they are most efficient. Instead, wings can use a little extra help at slower speeds, which is why they are typically equipped with lift enhancing features like flaps and slats.
Second, producing lift creates induced drag. You want the fuselage to be as streamlined as possible to produce the least amount of drag at higher cruise speeds, not more. And if your lifting fuselage is fully stalled at all other phases of flight it will be producing MASSIVE amounts of drag.
This full-stall drag penalty would create significant performance problems in all other phases of flight that would likely cancel out any small efficiencies at cruise speed, such as redesigned wings, more powerful engines, etc. I don't see anything to be gained by such an idea.
And finally, what makes you say that we are 20 years away from a blended wing body design being able to fly? There have been all kinds of designs built and flown in the last century, I don't think there are any technological hurdles to this type of aircraft.
Why adjust for lift only in cruise? Why not benefit throughout the flight envelope?
One aircraft that had a wide, lifting fuselage was the Lockheed SR-71 Blackbird. The fuselage was widened with sharp "chines" (a term taken from boatbuilding) extending forward from the tailless delta wing. During takeoff and landing the chines acted as vortex generators to provide vortex lift over the wing, similar to Concorde. During supersonic cruise the chines acted as fore wings to provide extra lift, stabilise the aircraft and stop the centre of lift moving back, as usually happens. But at high subsonic speeds the chines contributed little aerodynamically.
Some hybrid airships, such as the Airlander 10 have widened hulls to contribute lift, which of course only works as the ship picks up speed. But as far as I know, nobody has produced accurate numbers on the effect this has on cruise efficiency.
As another answer points out, a wide fuselage is structurally inefficient and tends to add weight. For large types such as the Boeing 747 and Airbus A380, designers found that they actually saved more weight, and thus improved overall efficiency, drag and fuel consumption the most, by stretching the fuselage upwards and adding a second passenger deck.
The problem with squashed fuselages is that they threaten a weight increase for little benefit. The fuselage, with its inherently small span, can generate little extra lift compared to the wings. The cost is that by reducing the depth of the very large beam that is the fuselage, you are exponentially increasing the amount of material needed to absorb the bending moment of the nose and tail.
In principle: yes, or well "mostly".
Though first lets tackle the elephant in the room: it's not the surface itself that generates lift - it is the geometry. And the obvious solution hence would be to deform the fuselage at higher speeds so it can create lift.
What you would do is have a fuselage that is "just stalling" when flying normally. This means air separation is just about happening in "slow" conditions. This means the airflow around the fuselage has just a little too little energy to stay attached to the surface.
If we then fly at a higher velocity the airflow around the fuselage has more energy. With the little increase of energye the flow could stay attached to the surface.
Now this is only a principle: since there is also the problem that lifting surfaces have huge hysteresis when going into the stall region and recovering. So the airflow would never reattach itself to the surface. You would have to add some active guidance to first attach the flow to the surface again.. - Or instantly being at large speed and never going slow in the first place.