As a writer, you always want to have an idea that is totally inspiring and most fun to work on, be it something very personal or something for mere entertainment. I say it should be as fun to write it as it is to read it in the end. Sometimes we have millions of interesting ideas for a book, not not all of those ideas hold equal weight on how they affect our urge to write. It's almost something you just know, when the most inspiring idea sends sparks through your mind ~ you just know it's a hit. Sometimes it's not exactly a plot, but a specific new style or experiment nobody has ever come up with yet. Maybe you've seen thousands of the same tropes but thought of a way to make something better and unique. Maybe it's when the stars align and every fragment of every memory for every video game, show, movie, and dream comes together to build a world you've always wanted to exist.
Either way, that kind of inspiration is critical. I've written one book and false-started another book before that I abandoned forever, not because the plots were too terrible, but because, I just didn't care enough to devote all my energy into those specific projects. You just have to find that powerful inspiration, and the rest should come naturally...
Unless you are struggling with writers block. I don't ever have that problem so I can't say I know what it's like. Wrong one to ask on that.
Fun Fact: Did you know that my Leray Series, before it began, was nothing more than a clustered mess of short story parodies and satire? I was just messing around, shootin the dirt, making fun of how so many stories have "magic" but never bother to explain it well enough to overcome the paradox of "having it here because, plot." It actually annoyed me that very few if any ever explained where magic came from, and then minutes later, I quickly adapted my own system in mind, and thought to make a magic series written more like a video game than a book (and no, not a Lit RPG or Iseike). It was fun for me because it meant I had to come up with all of the unique spells on my own, every factor, every variable, every name, every place and city, all stuff I would invent rather than assume, and have catacombs of mysteries and elements to that Leray world. As a avid gamer back in the day, it wasn't hard to take some inspiration from lots of media I had consumed, and I was careful not to Copy/Paste ideas or elements from that content; I wanted to make it my own, and so 19 books later, I did.
I'm rambling, but maybe this helps explain what that slump is and at least one possible vector out of it.