During one of our latest novel submissions for standalone novels, we received many Book I of trilogies. Here are some thoughts about the issues relating to that. We can't exhaust all topics, but hopefully it will help understand the implications.
Although we mentioned early termination of a contract, it is not the main topic here. Still, if you find that you cannot get out of a contract without financial repercussions and you have other plans in mind for your book, remember that you can always give formal notice that you will not be renewing the contract.
In the end, both publisher and author want the book to succeed - this should be a given.
Timestamps are available in the YouTube page of the video.
Enjoy!