I agree about Merle. I wish they had kept it a secret.
As far as what will happen with Merle...I sort of agree with your guess. My guess is he comes back, kills T-Dogg, and Daryl kills Merle. Regardless, Daryl isn't going anywhere. He's far too awesome. This is The Walking Dead though, so main characters go out all the time. That being said, I am expecting SOMEBODY in the main group to die tonight; I just don't think it will be Rick (obviously), Carl, Lori, or Daryl. Any of the others are likely.
I've heard Morgan is coming back this season, I am just wondering in what capacity and what purpose would his character serve other than just another gun for the group?
Yeah if he came back and took TDogg out it wouldn't surprise me, either.
Kirkman and Mazzara have already confirmed that there will be at least 1 main character killed off this season and hinted that it could be multiple characters killed off.
TDogg makes sense because he offers nothing to the show. There was speculation that he could develop into Tyrese from the comics but I think that role is headed towards Daryl.
I think another candidate to get whacked early is Maggie's sister. She's hot though and let's be honest here successful shows have to have a plethora of good looking people especially women. So her looks alone might be enough to save her. Another thing about her is the writers/producers may prefer to work with more experienced actors as the show becomes more popular.
People I think our safe:
Something about really successful shows are they have the ability to become even more popular once they kill someone important off. See: The Wire, Boardwalk Empire, etc....and then certain shows take a huge plunge once they kill important people off. I've always said that Lost was never the same once they killed Mr.Ecko off.