I'd be willing to bet the DLC will take place at the end of the actual game when Gabriel falls to his knees, etc. There's over a thousand year gap between the credits, so I'd say they will explain that a bit more, perhaps showing Gabriel's anger at god and leading to his transformation. I have a feeling we'll see Rinaldo at some point because of those scrolls mentioning he's alive, kind of interested to see what he looks like this time around.
Consider that Lords of Shadow during the game itself takes place in the year 1047 or something. For all we know said DLC could take place PRIOR to that Epilogue, building up events slowly to reach that point.
Nearly 900 years is a very, very long time for be explored.
Take example of GTA4 Episodic DLC while based on the same game engine are essentially self contained stories with new characters that expand on the existing universe.
Castlevania : Lords of Shadow could as well also have Episodes filled with original content and new story segments that could fill in the gaps for those period of 900 years or so.
For example the game could go into events just after the final battle when Gabriel starts to slowly transform and eventually become Dracula