> ... camber, slats, flaps, blowing of the surfaces

Unfortunately, they are some of the best ways of increasing Coefficient of Lift, far better than trying multiple wings, as seen with the Fiesler Fi 156 [Storch](https://en.m.wikipedia.org/wiki/Fieseler_Fi_156_Storch).

Deep camber and undercamber are [helpful](http://airfoiltools.com/airfoil/details?airfoil=dae21-il) in particular, as are endplates.  Another wing or two can be added. 

But now you have a very draggy, slow aircraft with very limited range,
unless you can *retract* these devices and speed up a little.

>Lift is proportional to V$^2$

If you have *even more power*, and the option to go supersonic, a short **single wing** like [this](https://en.m.wikipedia.org/wiki/Lockheed_F-104_Starfighter)
 or a **lifting body with no wing** at all like [this](https://en.m.wikipedia.org/wiki/Dream_Chaser) one would suffice.

It really depends on your fuel budget and available thrust.  Cost conscious airliners use retractable lifting devices, and slower speeds are required only because of limited runway length and the quality of your landing gear (especially tires).