Battery trailer falls under the laws of diminishing returns - you are pulling more weight, which means your car isn't as efficient, so you add batteries to make up for that, which adds more weight....
Generator trailer is always an interesting idea, and a relativly simple way to build a hybrid. It does make backing up and parking more difficult though.
For the Electro-Metro, I have considered adding a propane generator to a bumper hitch to make a Range-extended Plug-in Hybrid Electric Vehicle, which could also be called a REally cheap VOLT or RE-VOLT if you will.
One of my personal favorite transportation memories is from transporting bicycles on a bike carrier on the back of the Electro-Metro. I got passed by ANOTHER Geo Metro with a bike carrier on back. The guy was smiling and gave me a big "thumbs-up"!
Metros can move more than people give them credit for.