All signs point to X-Men.
The Marvel cinematic universe, which has recently become just about the biggest thing in entertainment (sorry Drake), has always had some notable holes in its roster, but now more than ever, with the huge cross-branding enterprises that are the Avengers movies, the lack of the X-Men and Deadpool is all the more glaring.
Yesterday, however, Marvel tweeted out a sketch of an obviously X-Men helmet, with the caption "X marks the spot." Aside from implying that there's something to find (ie. the 'x' on a map), the tweet is uncharacteristically cryptic for Marvel:
The drawing, according to Screen Rant, is by Deadpool creator Rob Liefeld, who said recently that "I have a new project, 2019, it's a huge X-Men crossover...It's awesome...Marvel called me up last Thanksgiving and said, 'We want you to do a big story.' I said well, let's see if they'll go for this, and I tried to do something no one has done before." The crossover he's talking about could be just about anything, but maybe it's the same one that Ryan Reynolds, Deadpool, has also been trying to get in the works by calling Hugh Jackman, Wolverine, "every day." There's also the fact that the rights to X-Men are owned by Fox, who Disney is currently about to buy out.