I think the answer to this is, almost anything. More characters, fewer characters, different setting, different ending, longer, shorter, rougher, gentler. People get asked for all of these things and much more. Publishers are trying to fit a book to a certain market, and they will want you to produce a MS that fits the market they have in mind.
How willing they are to work with an author to make a work fit the market they are aiming for will depend on how much promise they see in the author's work and how many other potentially publishable work they have in the queue. But actually publishable work from an actually publishable author is actually pretty rare stuff, so they can be willing to give a lot of feedback and ask for substantial revisions where they see potential.
But don't get the impression that this is about the publisher helping you to realize your vision. This is about you helping the publisher to keep product in a channel. Your scope for realizing your vision lies within the confines of that channel, and realizing it is largely on your head.