What about doubling the length of airfields in order to shorten the time it takes for a plane to roll on the ground after landing and before takeoff. The plane could take off from the middle of the air-lane and when it lands it can turn off in the middle, in order to minimise the driving time to and from the terminal. Why don't we see this and what does it take to achieve it?
The cost is that it would required a runway twice as long.
Airports already take up enormous amounts of land. Designers try to build airports near major centers, so taking up twice as much valuable land for longer runways near a major population center is a huge cost.
The longer runways also need to be cleaned, maintained, and snow-plowed.
That is a lot of cost and difficulty just to reduce taxi-time by a couple minutes. After one has taken a 2 hour cross-country flight, 5 minutes of taxi-time is not a point of concern.
Not quite to the same extent you have in mind, but when you are able to have a staggered set of runways, they will try to arrange it so that the "landing" ones have the roll out area near the terminal, and the "takeoff" ones have the threshold near the terminal.
Let's take BWI as an example. In an south/east flow (takeoffs and landing heading south/east), all traffic will take off on 15R, right next to the NW end of the terminal. All traffic will land on 10, with the exit from the rollout a little bit after the runways cross.
Or IAD, in a North flow, most landings will happen on 1R and takeoffs on 1C.
NAS (naval air station) Lemoore is another sort of example of this - kind of a little split because of the structure of the ramp, but if it was one terminal, in a NW flow you'd have all takeoffs on the N runway and all landings on the S runway.
Put another way, if you have a 9000 foot runway, in your scheme you'd need to make 1 18000 foot runway, but it is at least twice as efficient to have 2x9000 ones, even if it increases the taxi time, and some of that can be mitigated by the staggering above.
Notice that most airports are square-ish. For various reasons, it's usually a lot easier to get a compact parcel of land than a long, thin one. In particular, aviation authorities really don't like roads going under runways, so the airport as a whole creates problems for the highway system around it; double the length of the runways and those problems get much worse. In the old days, a square parcel was used to provide several (intersecting) runways at different angles, but many have been rebuilt to have parallel (and thus simultaneously usable) runways in the same compact area; taxi times might be a bit longer, but overall throughput is much, much higher.