I just re-read Blood Promise, because I am reading the series all over again, so that I could get a clue about what might happen. I read somewhere that Mead sort of 'leaves clues' in her books and if you pay attention to the minor details, you'll find them. And trust me, I did find some of them. Or may be it was just that I already knew what was going to happen. 
 Anyways, what happens in Spirit Bound clearly makes all of us doubt in Dimitri and Rose's happy ending, but just re-read Blood Promise, and you will see how much emotion Mead put into their story, which is why I don't think she'll separate them in the end. And sure, I like Adrian, he's our romantic bad boy and everythig, but, somehow, I see him with Sydney. She's got that 'evil creatures of the night' thingy going on, and it would be fun in the spin-off to see her going against her own rules and judgments. 
 Also, what kind of a role would Jill play in these series if she isn't Lissa's long lost half-sister? I mean, come on. Each and every person in these books has some kind of a role. Dimitri's family? Don't you have that feeling we'll see them again? Just like Eddie. You would think that after Frostbite, we wouldn't see him that much, and yet, he's one of Rose's best friends and partners in crime, even in Spirit Bound. Also, the card reading. Rose will probably get away from prison, I think. I would have to re-read Spirit Bound to be sure. 
 Oh, and Lissa will eventually become queen. Only with her on the throne we'll see the Moroi world as Rose wants it to be, with Moroi and dhampirs fighting against Strigoi. No, not just fighting, hunting them. That'd be really cool. 
 As in who killed the queen, it is obvious: Victor Dashkov. But he's probably had help from the other royals who didn't like her, like Adrian's father. May be all of them. That's why they all raised their hands in favor of Rose's guilt and punishment by death; to cover up for themselves. 
 Still, these are just theories, it doesn't have to be that way. And frankly, Rose's life has been either dedicated to Lissa, or to Dimitri. She might love Adrian in some way, but Dimitri's is her happiness, her peace, her everything. She deserves a happy ending for a change. Oh, and a book in which her life is being saved, and her happiness is attained don't you think?