Even Vin Diesel doesn't know! On Wednesday, on his highly active Facebook page, Diesel wrote (alongside a very dramatic photo of him jetpacking) that "Marvel has requested a meeting no idea what for haha, you probably know better than me "
Collider speculates: "The studio recently announced firm release dates for three upcoming films, two set to open in 2016 and one for 2017. The general consensus is that Marvel will reveal the identity of one or more of these films at Comic-Con next month, and it’s possible — though maybe not likely — that they’d like to announce Diesel as the star of one of these at the panel as well. The presumed “Big Bad” for Marvel’s Phase Two is Thanos, with the character likely acting as the primary villain in The Avengers 2, and it’s not exactly a stretch to think of Diesel taking on the role."
And while, yes, that'd be awesome and all, it's also possible Marvel has requested the meeting just to vigorously pump Vin Diesel's hands while congratulating him on providing us with a cornucopia of high-octane entertainment. You know, he walks in — boom, balloons, streamers, banners: "You're the best! We appreciate you! That scene where you and The Rock both air-tackle that dude on the plane in Fast Six was so next-level!" That kind of thing. Anyway, stay tuned — and keep refreshing Vin Diesel's Facebook page — for more.