There are some people I've seen on Twitter saying "Why don't they just reveal them already? We already know they'll be there."
But then, you ruin the big moment of seeing it for the first time in the actual theater!
Cap wielding Mjolnir or "Avengers Assemble!" wouldn't have had the same impact if they gave that away in the trailer!
On the other hand...they could just make the trailer better than the movie.