Great post. AMC is probably trying to hold onto as much of the viewership from Walking Dead and Breaking Bad as possible.
Here's my two cents of the Walking Dead Spinoff and what I think will determine its success:
*It depends upon which characters are in the spinoff.
If you go by the traditional corporate entertainment formula (where past success dictates future), AMC may use characters that have shown to be successful in either the comic book series, novels, webisodes or the original show.
If that is the case AMC may very well bring a large chunk of their Walking Dead audience over to that new show. They then will be able to throw more money at the budget because they will be able to get more money for the advertising space in the commercials.
Some candidates from the original Walking Dead could be: Carol, the Morales Family, Morgan or even Lilly and Tara. From the webisodes it could be Lori Grime's brother Chase.
All those folks could easily lead hard core fans to the steps of the new show.
That being said, Robert Kirkman has indicated a few months ago that this new show will contain only new characters for the new series. In either case good characters will make or break it.
*It depends where the story is to take place.
The original Walking Dead is set in a very specific area with specific characterization.
I agree with Robin. AMC literally could take the show anywhere and keep going with other characters.
It's wide open. However, if they are budget conscious and there is shared properties between the two shows it may also restrict the location choices to Georgia.
Some locations would be of more curiosity then others. For instance, major cities, government facilities, public works (dams, power centers,etc) are definitely more interesting places to tell stories over a small town in the country side.
Robert Kirkman has indicated it won't take place in Georgia.
*It depends who is in charge of the project and the overall goals of the spinoff and the writing choices.
Will the spin off somehow feed into the main show?
Is Robert Kirkman involved as far as show writer?
(The answer to that question is most likely yes. Kirkman loves his universe and will probably be there on set.)
What comes to mind here is the second season mini arc of the TV show 'Lost' with the Tail section survivor episodes. All new characters that complement characters already introduced (In the case of the TWD spinoff, Lori's brother Chase would do well if this is what happens.)
*The Robert Kirkman factor.
The thing also to bear in mind is the mind of Robert Kirkman. He has been very clever about how he has handled his universe. Each medium: TV; comics; webisodes; and novels have different characters; storylines; and action beats. He's very excited about the possibilities in each medium and it shows.
As an example, to me Carol in the TV show is more like Andrea in the Comic book in terms of her personality and behavior.
He has used bits of the governor novels for the governor episodes in the first half of the season four of the TV show.
I can tell you as a fanboy (having read all 118 comic books to date, all the novels and seen all the walking dead shows and webisodes) I have never been quite able to predict where the TV show is going exactly. That's what I love about the TV show. (That also applies to the comic books, novels and webisodes. I have always felt the way the characters in each of these mediums have felt when they get surprised in a new situation.) If the new show is the same way then I am definitely watching.
All of these things are equally important, however I put them in the order of what I believe would bring about the most success.