The way to make a plane do more MPG is... wait for it ... to make it smaller
Don't disagree, but also compound curves, reduced weight, larger aspect ratio, winglets (when appropriate), smoother surfaces, and some consideration towards reducing parasitic drag. My understanding is that the Long-EZ is so efficient in large part because of its decrease in surface area, because the fuselage volume is so much better utilised.
As an example of how older aircraft simply haven't been designed with efficiency in mind, I always wondered at the sharp wing-root junctions on the PA38 and PA28. Sure enough, when I looked for a wing root fairing kit, the manufacturers claimed to increase speed by 2-3 knots - which probably counts for an efficiency increase of 5% or so if you kept the airspeed constant.
I still suspect that many of the drawbacks of the present offerings are due to the microlight and LSA design criteria. They are mandated to have comparatively low stalling speeds, necessitating larger wings. Top speed is limited, so they'll never be able to compete as tourers.
But anyway, where I live my renting options are essentially C42s or PA28s - why would I want to hire a 4 person plane for a solo or 2-person outing, which will make up the majority of my flying over the next few years.