Great news. I look forward to Thursday.
I'm guessing it will be the "first" trailer of a possible 3, considering the actual first trailer is a like a teaser or an introduction to the world. I think we'll see more of the environment and maybe an introduction to a character, maybe the/a protagonist with a little bit of a story tease. I'm not expecting it to be too revealing but we know it will be picked apart on here.
That leaves a possible two more trailers between now and the end of Spring. One may focus on story and the other on gameplay. Maybe there'll be a 4th after the 28th to showcase online. With the criticisms of V's arguably short story, I wouldn't be surprised in there's emphasis on the singleplayer experience. As we know RDR was praised for it's narrative and I think it's definitely something Rockstar will want to show off, that despite GTAO, RDR is very much a going for rich storytelling in a compelling environment.
I doubt it's an announcement for screenshots. If it was for screenshots, I reckon they would have just released them unannounced.