Spoilers
At this point, very little is known about the plot of the movie, but because of the fact that it has a long history on television and film, several things can be assumed.
Note: before you continue, this is for people who absolutely can't wait to see the movie in theaters and want to explore the secrets of Collinwood right away. If you some surprises left intact, go to another page now!
The first and most obvious (but depending on the story or how the movie is made, it may not be obvious at first!), Barnabas Collins is a vampire. Yes, he's the same Barnabas Collins from nearly 200 years ago, even though he claims to be merely his ancestor. Sometimes this is obvious from the very beginning (usually a common thief (Willy) frees Barnabas from the coffin and it's pretty obvious what Barnabas is and from earlier foreshadowing who he is) and sometimes you're left to wonder a little bit.
The little girl, Sarah, if she appears, is actually Barnabas' sister from nearly 200 years ago. She died when she was 9 or 10 from a mysterious illness, and when she finally starts to appear, she seems to be working against Barnabas.
Dr. Julia Hoffman falls in love with Barnabas. She will most likely act in a fit of jealousy to either destroy him or impede him somehow.
Victoria Winters is really Elizabeth Stoddard's illegitimate daughter. She was birthed in secret and sent far away to grow up in a foundling home. Now, either out of guilt, curiousity, or even love, Victoria was brought back into their lives. Vicky doesn't know who she really is.
Regarding the above, it's possible that Victoria Winters may not even be used as a character. The first Dark Shadows movie used Maggie Evans as the governess to make the plot simpler and to focus on Barnabas.