Most publishers like to know there is a series possibility with a project but that it can stand alone. I encourage people when they first start writing a project to decide a series concept for a couple of following books right then. You don’t need more than a nice sized paragraph on the overall concept to put in a “series potential” section in your proposal, but it’s good to have it right from the beginning and it’s even better to have the concepts in the back of your mind from the beginning.
For one thing it can help you set up a following book as part of your writing task if you have them in mind. You may have ideas that do not fit within the project that you are working on but might fit in one of the other books and you can toss them in a file for the book to use if and when you start on it.
You can make sure you have the necessary characters for series books in the first one and can make sure they are set up correctly and can make sure the over-riding theme is established. Doing these things will add depth to the book that you’re working on even if the series does not materialize and it is fated to be a stand alone.
I told one lady not to use the word ‘trilogy.’ A publisher is then being presented with taking three books or none at all. If they are presented with a stand alone that has series potential they can take the one book, they can decide to take all three (and maybe get the second and third cheaper than if the first book is successful) or they can take the first one and option the other two and wait to see if the first book is successful. Or the publisher can decide to do the book as a trilogy. I prefer them to be the one to decide that instead of pitching it that way. Giving them these choices makes the project much more attractive to an editor.
A publishing house invests money developing an author, and they are more open to doing that if they can be sure there is more in the pipeline to justify the effort and expense.
Should a book be pitched to have series potential? Absolutely.