It may not be a completed successor mind you. However, from the look at the engine frame on this Veyron, it would appear that Bugatti may be at work on their future.
There have been a lot of rumors at to what the future holds for Bugatti. We know not to expect anything until 2015, according to President Wolfgang Schreiber. This makes a lot of sense, considering that as of February, Bugatti still had an $84 million stockpile of the old Veyron's to be dealt with.
We know not to expect a SuperVeyron or Galibier, and there is a slight possibility of a hybrid in consideration. We also know that Bugatti is probably looking at the available technology in materials, and deciding how they can shed several hundred pounds off a Veyron successor. This is assuming they will be going with the 'bigger is better' rule when it comes to its powerplant choice, and I think that is a safe assumption.
Speaking of engines, in the video below, Youtuber, Fastsportscardriver caught a glimpse of a Veyron roaming Nurburgring with what would appear to be a frame fitted around the engine bay. While there could be a lot of explanation as to what was going on. I choose to believe that Bugatti is already working on a new world's fastest supercar.
It's either that or Back to the Future Part 4. See it in action in the video below: