Hawkeye would be a very interesting choice and he is at the one I voted for, but there are a few other possibilities. The Wasp would certainly be interesting but I still having a hard time; she just seems kind of out of the loop to me, like she her position in SHIELD just isn't prominent enough.
I have a bad feeling that Black Widow will be the one, but she just seems too obvious. I wouldn't neccessarily be disappointed if it were her, it just wouldn't be much of a suprise.
Then there's Scarlet Witch, another possibility. While reading #5, I developed a theory that Scarlet Witch was either being impersonated by or controlled by Loki, hence the reality warping and the urging on her brother. Before I read the issue, I had heard someone else suggest something like this, and I feel that it was dissmissed too quickly.
Pym is a possibility, but is too obvious. I was interested in the Banner theory, but Bass's comments made me realize it wouldn't work. Cap: no way. Tony: not very likely. Nick Fury: possible, but I really don't think he's a villain.
As for the possible death of Hawkeye's family: How do you know that the traitor is going to kill them? It could be an indirect result of the traitor's actions.