Blizzard is working on something unannounced and Overwatch-related, of that much we can be certain. What that something might be, however, remains a mystery.
We caught wind of a job advert for an internship for someone to work with the art team on an unannounced project, for which they'll be creating "hard surface and organic props, weapons and possibly characters that are both detail-oriented and consistent with the art style we've defined."
Later, in the same advert, it stresses that applicants will benefit from having knowledge of the Overwatch universe. Naturally, this desired requirement combined with the words "unannounced project" have people theorising.
It could be something as simple as an upcoming and still unannounced event set inside Overwatch itself, although there's always the chance that it might be a mobile game or something along those lines.
Blizzard is certainly working on something, but given the popularity of Overwatch, that's hardly a surprise. At this stage your guess is as good as ours, so let us know your theories in the comments below.