For quite long trips a gas/diesel generator would currently be a better option giving longer range and utilizing the current infrastructure. Battery trailers would be good for moderate trips but not long trips with current technology.

"Long trip" in Europe is different than "long trip" in the US. A long trip here could be 2K or 3K miles with lots of nothing between buggy locations. I'm with you that a diesel genset would make more sense here.

Sorry, but I predict doom and failure for this idea.
Firstly, no-one can drive with a trailer attached. Backing, parking, going round corners, changing lanes on the freeway are just going to result in a lot of accidents.
Secondly, the added weight of dragging that thing around is going to kill your efficiency.

This is actually a really good idea. People that own electric cars own them as second vehicles only and something like this would enable them to be used as a primary vehicle because it would extend their range.
One of the biggest problems with using an electric car on long drives isn't just their shorter range but because they can't quickly be refueled. Being able to replace the main battery pack in the vehicle requires a bunch of difficult problems to be solved first but this would be compatible with anything.
and @Max, if you owned only an EV and needed to take it somewhere where only gasoline would be available you could always trailer a gas/diesel generator the same way.
It seems kind of stupid to trailer a gas powered generator behind an EV but the point is you would only need to do this a couple times a year (like on a camping trip or something) and you would still be EV only the rest of the year.

Why not set up the trailer as a generator, driven by the trailers tires, that could provide a continous flow of energy to the prime mover while travelling at highway speeds? Could also be made so that the trailer would go into neutral at lower speeds with the energy provided by the battery pack in the prime mover (in and around town situations).
I'm not an engineer of such things so there may be something fundamentally wrong with the idea but it seems this would be easier to implement than an infrastructure to provide trailer replacement support with sufficient saturation to where people would feel comfortable making long trips with an all electric car.

Why not just fit cars with a replaceable battery pack like a forklift?

Better than the Dock Go but still not as good as an ICE driven vehicle.
If I am going to deal with the hassles of pulling a trailer I want more than a gas tank. And what's with that wing?

You're basically proposing a perpetual motion machine -- or free energy. It doesn't work that way. You can't get more energy out of the moving wheels than the energy you put in to move the wheels.

@Rt1583 the fundamental problem with your idea is that where does the energy come from that is driving the wheels of the trailer?
On the flat etc, it comes from the car.... so you'd just be using the car battery to charge the trailer battery via the generator / wheels / road / wheels / engine.
If it only worked as a brake (e.g. hooked up to your brake system) as an energy recovery system, that would be different, but these are generally built into the car anyway.

So that's "unlimited range" as long as you stop to refuel every now and then? I'm no expert but I'm fairly certain my non-electric car works in a similar way.