It depends on - assuming there is something waiting in the wings - what the hold up is. An announcement four days before Xmas isn't going to hinder Matt Smith's final episode.
On Dec 21 there's a BFI (or was it MBW?) event. Some think it could be announced there.
OR if it is Marco then February next year would be a good choice, as that would be the 50th anniversary of The Roof of the World was broadcast.
They definitely wouldn't hold it until August.