Just a running commentary/brain dump as I read over the OScar page from having had similar thoughts myself for a long time... some requests, some "long term future" ideas, and some stuff hopefully of use right now.

Supporting multiple sizes a plus - this could include 4 passenger or possibly even more on a trike design still... in part because in many places trikes are taxed or insured as motorcycles which can have lower costs because of it.

If a 3 wheel version and a 4 wheel version could support by and large the same parts it would be a plus. The aerodynamics of a 4 wheel doesnt have to be worse than a 3 wheel.

Choice of bodies i'm hoping is an option... ideal aerodynamics is best for long range. Others may need/seek something akin to the new 'boxy' minicars like the Scion xB for urban use.

Shared designs between a hydraulic accumulator and electric plug in hybrid capable would be a plus if possible... for urban use the latter may be more suitable.

Use of gasoline, diesel, or other alternative fuel options could be a plus... for instance a producer gas vehicle is going to require some notable changes. (all the above comments are just "flexibility would be nice" is all... parts only have to be identical for mass manufacturing, if the goal is individual empowerment then it may be worthwhile to design in as much flexibility as we can reasonably create)

On the subject of EFI, I would imagine MegaSquirt could work just fine http://www.megasquirt.info/ it can run 1 cylinder engines and be adapted to anything. Use of junkyard sourced sensors and such would reduce cost vs new, but it will still cost a few hundred dollars to put together a system. If this means more flexibility in engine choice it may be worth it.

For converting a vehicle to alcohol or E85 the only changes normally are the avoidance of certain types of rubber I believe which degrade in the presence of ethanol or methanol, and often raising the compression ratio to take advantage of the higher octane of alcohol and to gain back some efficiency. (source: hot rodding reading and knowledge including of others conversions to alcohol for performance reasons though normally in combination with gasoline fuels and not pure) Mileage will always be less on E85 and alcohol even with higher compression ratios, it's main advantages are for high power output (the way it absorbs heat lowers combustion charge temperature allowing far more turbo/supercharger boost) or perhaps renewability. Sometimes carburetors need internal modifications if they use one of the rubbers that degrades. Pure alcohol in cold climates has alot of difficulties which is why E85 is often as high as you want to go... but a vehicle designed for E85 could usually run E100 with no modification, the 15% gasoline just helps cold weather starting and such.

Eventually having some kind of automated software which could design frame members to attach the front end and rear end components of ANY available donor vehicle to match up with the rest of the design would be nice... (longer future) The ability to use what you have laying around or can find for lowest cost in good condition or to take advantage of market quirks. I have an idea of how such software could be designed although i'm not a programmer...

Support of higher HP figures could be advantageous - even using 20hp or 35hp will have mileage benefits over a normal car with 75-100hp in the smaller classes and could allow things such as towing a trailer (probably need 4 wheels for that) more efficiently than any other tow vehicle. For that matter I have almost more interest in a microcar actually designed to tow a good 2000lbs or more so I can throw a trailer behind it if I need to move stuff (giving 4cyl pickup style usability) but normally used as a daily runabout. Such a vehicle would probably need 50hp or more in a hilly area to work well, at worst being closer to existing small car mileage figures but with the haul abilities of a small pickup or the trailering of a small medium sedan when needed. One wacky idea might actually be a powered trailer for use in such situations... a 2nd engine, the Power Cube from elsewhere, perhaps with an accumulator or for simplicity (and not often used) without driving the trailer axle itself, but attached and controlled by the OScar, this would mean no mileage compromises when driving without it, and the trailer still licensed and insured as a trailer instead of a 2nd vehicle for cases where that is important. (like low income individuals who they discourage from having more than one vehicle)

The adaptions to run on ethanol can just as easily run compressed natural gas or propane as long as you have a gas mixing carburetor... all three use a higher compression ratio than normal gasoline and the latter two may be more appropriate at times. For instance fill at home CNG stations would fill a small car tank like the OS car in a few hours at a cost 1/3 that of gasoline and far less than even subsidized ethanol, only making your own ethanol could possibly beat it. However to add a gas mixing carburetor like this alongside EFI requires a separate controller normally... however producer gas is special, although I seem to remember it having higher octane/liking higher compression

Modifying many small engines for higher compression is difficult... overhead valve is at least needed to get any reasonable compression ratio and most of the time a replacement custom piston, often domed, although sometimes milling the heads down can raise it by a small amount (usually not enough for what alcohol will want though, like 12:1 or greater instead of 9.5:1). Such a piston may be found at lower cost if you used an engine already used for go kart racing for instance, or some other place where performance modification occurs on such small engines. (thats the only place I can think of offhand) Otherwise a custom CNC machined piston is required.