A new Google Chromecast was announced in the "Federal Communications Commission" FCC Link bringing only hardware improvements over.
These documents show of a minor hardware modification by the name of H2G2-2A Revision.
A couple days ago 6 of the members of the Chromecast team joined the new Reddit AMA "Ask Me Anything" initiative an created the Ask Us Anything about Chromecast where according to Google Jacky Hayward:
"We don't have any new user-facing features planned for this device."
Anyway changes will be done to the new model:
"it will have the same features as the original and most users won't notice a difference."
So will have to wait to see what will come.