********Spoilers
Wow is right. That was a hell of a twist.
Being speculative here on my end, I think Deb is going to keep searching and finding out more info for that part of the Dexter storyline to progress and have further meaning. I cannot say that she'll discover that her bro is a serial killer any time soon, but I think they will develop that tension and growing suspicion about Dexter from her POV. In the books, if I remember correctly (I haven't read them myself), I think she knows about Dexter's killing habits. And there is definitely a path laid out for her to discover that, but how far they take it is anyone's guess. Deb definitely acts out according to her "hunches" as well, so I could see that eventually leading her to question Dexter more and more as things progress.
Also, there is now the obvious perfect setup for Dexter's kid to have some darkness as well now. Yes, the kid is younger than Dexter, but I have a hard time thinking they'd won't eventually play out that bit regardless. Perhaps not so much in this coming season, but maybe in a future one, the setup is there and in place. Maybe the kid will show some signs of it already at a very young age next season, even if it just some type of uncomfortable smile at violence that is on TV or something like that, but not a full blown part of the story with Dexter teaching the kid how to tame her demons. Leave that for the season after the next. j/k

With the going forward direction of the show, it is hard to say where I think they should go with this. At first I thought that they should pick things back up with Rita's killing, but there are limitations there, with Trinity already out of the picture. The FBI are in some way going to be going after Trinity, but it's a dead end in some respects. Though the FBI will be looking into Dexter's arena now with Rita's death, depending on how Dexter handles it, and I don't doubt they will probably end up doing that.
Something with Dexter and Quinn has been growing, so you've got that going on too. Quinn is going to get himself in trouble one way or another before it is all said and done. A different kind of annoyance for Dexter, as opposed to what he dealt with before with Doakes.
LaGuerta just needs to die, but Angel is still cool in my book.
Anyhow, Sept 2010 is too far away.