I'm enjoying watching the second season of Dexter - I loved the first season all about why a person kills and it got me hooked, though this one has more on morals and about the inner turmoil of the mind of a killer.
I've yet to read the books, though I’m a big fan of the American Psycho book, and love the nods within Dexter and its similarities. I understand there are books which each season is based upon, so I’m guessing the end would be decided by Jeff Lindsay.
Though in the name of fun, here's what I’d like to see;
1. I agree that it would be interesting to see Dexter in prison - it would be fun to have him snap and basically go on a bloody killing spree killing all the inmates before killing himself in order to escape the death sentence he would likely receive.
2. Deb would freak and would be unsure on her feelings about if she should help Dexter or let him rot - and she would be especially mad if she knew how Harry helped him. Rita would no doubt blame herself and her inability to pick a "good" man. And although you don't include her I think part of Lila might like the destructive element of Dexter.
3. Again I agree if Harry never found Dexter someone else would have (i.e. no set of "rules" or moral guide lines to follow) and him and his brother would probably have grown up in and out foster homes before becoming the worst (or is that best?) serial killers in history.