This is just my opinion, but I think that Marvel and the Russo's are making a HUGE and concerted effort to keep the release of any and all information to a minimum. They know that there's a rabid audience just chomping at the bit for anything they can get and their plan is to string out that desire as long as possible.
Take note of this trailer. It certainly lives up to the billing of being a "Teaser". While D.C. gave away virtually all of Batman v Superman and Justice League in their trailers, Matvel has given almost nothing in this one. Filmmaking 101, get them to go see the movie to find out what happens.
This trailer showed just a few shots of the original Avengers team, that's it. Hawkeye as Ronin was the "big reveal", and you didn't even see him in full costume! Not only that, but they've didnt even show all of the heroes that survived. No War Machine, no Rocket either. Prett bare bones. Heck, you didn't actually even see Thanos!
Half of it was Stark's recording while drifting through space.
I'm not complaining, but they really didn't show their hand much at all. I see a deliberate, slow burn at work.