My issue is talking to the group and just saying "Hey, here's X idea I had to kick things off, does that sound cool" could go a long way to ensuring everybody has a good time. If I was pitched "a Dirty Dozen kind of deal where maybe my guy is a criminal, maybe they got the wrong guy, but we're given this suicide mission to clear ourselves and square things", I can think of a couple of ideas off the bat that I'd enjoy playing. But if I didn't know that going in, it might really throw me off.
I question that, assuming you want to do it, there's much value in it being a surprise. It might not be a good surprise. And you should feel comfortable enough in your campaign and the group that people are going to try to have a good time and get into it, and they don't need a big sudden surprise (that could potentially be interpreted as a bait and switch and just be kind of a mess, or not, but it's a risk).
I question that, assuming you want to do it, there's much value in it being a surprise. It might not be a good surprise. And you should feel comfortable enough in your campaign and the group that people are going to try to have a good time and get into it, and they don't need a big sudden surprise (that could potentially be interpreted as a bait and switch and just be kind of a mess, or not, but it's a risk).