This is surprising news. I had heard that Matt was contracted to play the part for at least the next one or two series.
We might see a female doctor, which would make the River Song relationship interesting.
One of the things they could do is write several story arcs where the doctor regenerates. You can feed a lot of actors through the part in the next few years and make it very companion-centric. This would be similar to Spinal Tap drummers.