When fans tell you how to write your book, do you listen?
Just kidding. Sort of. Mebbe. There is no greater compliment than fans who really connect with your work. Some of those fans will shout their passion from every review page, book party table, and fan-Con hallway (gods bless 'em; most authors really need that visibility boost). Similarly, everybody has an opinion. So when those opinions about your work collide with a passion for the characters, world, and plot there might be a few "suggestions" made.
Don't kill that character.
Ship these two characters.
Get that character some religion.
Not that religion.
Speaking practically, in a series where books are still being written, the author is most likely writing two or three books ahead of what's just hitting the shelves. A lot of those "suggestions" are too late. Similarly, the series may only have been contracted for X number of books or the series was sold as a trilogy so there are certain plot points, milestones, character developments that have to occur at a specific in the timeline in order to satisfactorily end the series. Again, the suggestions are too late.
Now, what if, what if it's not too late? What if--because the author is a super-slow writer publishing on their own damn schedule like moi--there's time for fans to voice an opinion and have it be heard? Well, the kind of feedback that might sway me is if a minor character turns out to be a fan-favorite; that character might end up with more page time. Not so much as to take away from the story, but sure, why not give a nod to the fan base? Plot suggestions? No, sorry. My world, my crazy. Who lives and who dies? Sometimes those developments surprise even me. Who's going to get laid, when, and how explicitly will it be depicted?
Nookie will happen if and when it's meant to during the characters' developments. Whether it happens on page or off depends on a lot of things. It's not a radio show; the characters aren't taking requests.