Opel has already confirmed that an Astra Cabriolet model is coming and spy shots here show a rear end consistent with a convertible model.
Here are a few shots of a possible Opel Astra convertible that has been spied.
We don't get to see much as the car is sitting in a dark garage and has its roof covered with a tarp. But the boot lid has a small built-in spoiler on it, unlike any Opel model we can think of, and that roof has a steep angle to it.
The top image here is a rendering of what the model could look like.
We can expect to see a convertible along these lines emerge soon enough as Opel has already confirmed that an Astra Cabrio is coming in 2013.