The only thing official pointing to TBC Classic was a survey emailed out a while back asking questions regarding it. But other than that Blizzard said a word about what happens after Naxx. Nothing has been announced.
General consensus is that the "classic" period of WoW is Vanilla through WotLK. So it kind of makes sense that Blizz is probably looking at doing these 3 expansion at minimum as long as classic stays populated and profitable.
We are supposed to have some sort of Digital Blizzcon early next year. I am betting we will get some news on whats in store for the future of classic. If Blizz follows the release schedule of Vanilla like they have been Naxx Will be out by the end of January so people will be chomping at the bit for news.