I saw an article that said they were casting to men in their 20's and a girl too. So some are saying probably Jania, Jacen, and Anakin. it's also said it could be the set up to the Yuuzhan Vong war. but it's also said that they aren't gonna base it off of the EU, but who knows. and it's not confirmed to be called "A New Dawn". just rumors, but it most likely.
I do not know what the base of the story of the new dawn. The story might be base around the new Jedi order and the kids of skywalker and solo but knowing Disney they will keep the story line a secret until they start the tv series rebel which should be in the same time line as the new movie or leading up to the movie.