The future dlc's for fallout 4 are most likely completely new. But what if there based off of the Fallout 3 dlc's? Well if they are, than i will try to speculate what they will be about.
1. broken steel.
in the game play trailer, its very clear that the brotherhood is the enemy. and if you somehow die at the end of the game, you end up with some faction, probably the minutemen because Preston have been seen fighting someone in the concept (maybe the brotherhood). And you have to rid the brotherhood of boston
2. point lookout.
http://www.gamesas.com/topic/1528354-is-long-island-the-new-point-lookout/
not gonna spend much time here, just one of my previous titles.
3. operation ancorage.
the concept released on the 4th of july
http://cdnstatic.gamesas.com/bethblog/2015/07/MoF_MURAL1440px.jpg
maybe you expierience the battle we sea in this concept.
4. mothership zeta.
we saw concept of them, i have no idea, maybe you find them on the moon battle?