We haven't had a mysterious Marvel teaser in a while. Today, they make up for it with the following vague teaser:
The only text accompanying this teaser was the following:
The Marvel Universe is on the brink of a major change. Find out more, September 18th!
Looking over the releases for September 18th might give some clues.
In CAPTAIN AMERICA, we'll see a new arc beginning but since it deals with Nuke, that's probably not it. There is the INFINITY HUNT miniseries beginning also. That doesn't seem it either. What about the new arc in THOR: GOD OF THUNDER? According to the solicit, Malekith the Accursed is returning, just in time for the movie. Or maybe it's about ULTIMATE COMICS ULTIMATES #30 dealing with the revelation on the She-Kang character.
Perhaps Marvel will be making an announcement on that day and this doesn't pertain to an actual issue coming out that day?
What's your guess?