Agents and Publishers like to hear that a want-to-be author intends to write more than one book. If a want-to-be author intends to write only one, then they may fail to hook an Agent or Publisher.
I believe the intention to write a trilogy or a longer series would be a much more attractive prospect to an Agent or Publisher.
I have roughly outlined 3 more books in my series. I have the beginnings, middles and ends to them. I have written many ideas for the storylines and plots, but I have yet to add the working chapter titles and the key landmark events.
From the outset I intended to write a series of novels using the same characters. It is easier to write a sequel or subsequent books, without having to define the differences in your characters and their functions.
Detective stories with the same protagonist are a prime example. Why kill off good characters when you can recycle them? I believe that many successful traditional and contemporary authors would have been wiser to leave some of their characters in the ether.
Do you intend to kill your characters off?
David, firstly let me say that I loved your formaldehyde comment (I'm not sure what that says about me, but hey-ho, it made me laugh anyhow.)
Secondly, to answer the question, Adrian:
In my case the death rate in my stories is fairly high anyway. (I haven't tied it down as tightly as the Star Trek "People In Red Jumpers Will Die Before The End Of The Scene" rule, but it's almost as bad in places.)
Of the major characters so far, two had to die otherwise the story would not be able to take place. They expire in the prologue. I can't really call them "Expired Extras" as they are intrinsic to the plot, but they're not really main characters either. Sometimes I think of them as a class of characters that I call "Perishables".
Eventually I will have to kill off at least one lead character so that his apprentice can take over the reigns from him. But hopefully that will not be for quite some time yet. I would like to give him his own series and then continue with his apprentice. I suppose when I feel that he's done his bit and the apprentice is ready, that's when he'll pop his clogs.
Extras are always dropping like flies anyway, so I can't see the death toll lowering any time soon for them.
I think a few necessary deaths really bring action and suspense to a book because suddenly no one is safe. I also would love to see the reader's face when a totally unexpected character gets killed - I personally love books where you don't know what's around the corner for who ever, even the main characters.
Then again, people can overkill (excuse the pun) when killing off characters. Too many deaths is a bit tiring and not as emotional as the first few.
Personally, I plan to write a five-book series with a minor character death in three, a large in four and two large in five. Even I think that's too much.
I think it depends on the kind of book you're writing - fantasy and sci-fi get away with it more than romance or drama.
No single book interest?
Look at it from the commercial point of view...
If someone brings you a text that you are sure will make everyone a fortune - are you going to blow that opportunity away because the author doesn't have either a second book planned or even any inclination to write one?
As to killing people off... As 99.9% of the world's population has already died in my scenario most people are struggling to stay alive... So far it is (mostly) bad guys that I have killed... So far...
David