Along the lines of more fuel efficiency, though that's Not the only subject to be discussed here:
Duke Engines 5 cylinder rotary engine. A strange design with some upsides and some flaws. Higher compression is a good thing, since that's far better efficiency. But cooling, in particular, would be difficult to accomplish. Might want to add additional cylinders so there's a cooling cycle, and water injection would also help, as that is used in airplane engines. Yes, that sounds weird, but I'm not joking. Airplanes also have dual plugs per cylinder so there's additional power and a backup in case of failure of one system.
The Duke engine reminded me of the OPOC engine, whose website sounded like the good doctor had taken Bill Gates' money and ran away. Turns out not to be the case. They're still moving forward, having gotten manufacturing contracts to build it in China (of course), and they've got a new CEO and CFO so it will stay in business. I really hope the actual production model gets out there and running so we can learn if its biodiesel compatible (as claimed), powerful, and in scooters (as planned). The 50cc 2-stroke model should run like a 200cc four stroke, which is plenty of power for a scooter. Two of those using the intended clutching system and necessary connecting bits would still be a dandy motor for efficient power for cruising and climbing. I'd love to see one of these climbing the Sunol Grade or Donner Pass while still smelling like fry oil. Lots of potential. With $33/gal diesel coming, sustainability will be about the compromises and this would work. This sort of engine is being made, or was originally planned, in three sizes: 50cc, 400cc, and something bigger for ships and trucks. The modules can be attached to others and electric clutches allow them to turn on and off without operator input other than pressing the accellator or RPM demand. Hit a hill, the second engine starts and runs till you crest it, then shuts down again, saving fuel. A nifty idea, if it can work reliably. The big problem with lots of moving parts is they are more points for potential failure, and more maintenance is required to keep them working.
I post these links and comments because as cool as they are conceptually, they aren't proven in the real world, in real driving conditions, and roadside repairs take years to percolate down to the masses. Conventional 50cc 2-stroke scooters have been around for 120 years and get 130 mpg with carburetors and standard poor quality gasoline. The OPOC and such are interesting, but almost certainly more expensive. When they cost $40 like a cheap Chinese 50cc scooter engine does, I'll be more impressed.
Good scooters with good reputations for real world riding: Piaggio Fly 150, Aprilia Scarabeo 200, Aprilia Sport City 125, Yamaha Zuma 125, Kymco People 150, Kymco Agility 125. I'm leaving off the Vespa, Honda, and Genuine/Buddy scooters because they're thousands overpriced. Note that I'm ignoring the 50cc scooters because they won't climb hills and half the USA is mountains and hills. A 2-stroke 50cc will do it, but most are not clean enough for EPA and will probably be banned in the near future. The bigger scooters (200cc+) cost as much or more than a proper motorcycle, which is more difficult to operate but better in every way for safety, terrain handling (suspension, wheel size), and have similar or better fuel economy than the big scoots. There's little point buying a 250cc scooter for $5K when $4K can buy you a decent 250cc motorcycle (Honda CBR250R, Suzuki TU250X, Kawasaki Ninja 250) all of which are proven machines in the real world. Remember that bigger wheels are safer, and the tires are less expensive too. I'm reasonably sure that paved roads are one of the things we'll remember fondly, like ice cream and cold fizzy drinks in a disposable aluminum can. Someday our beer will come in kegs at the local brewery and big glass bottles with a hefty deposit. Laugh now. Go ahead.
Someday we'll be driving 3-wheeled trucks a couple times a week, or paying those who have them in eggs or beer or a box of veggies for their loan to pick something up or drop something off. Our lives of indpendence and convenience are ending. Getting along with neighbors will be the hardest thing we've had to do in our whole lives. I think we will find that independent living was the real luxury of the 20th century.