One of the reasons I scoff at prospective clients who offer me a percentage of royalties to develop their ideas into full-fledged novels is that ideas are plentiful and free. Developing them into something fit for public consumption takes time, effort, and skill. It's hard work. Not physically difficult, mentally difficult.
I don't exactly build muscle sitting at my computer tap-tap-tapping on the keyboard.
If the "experts" are right, there are only a small number of overarching plots anyway. Every story is based on one of those archetypes. So, the prospective client's idea isn't exactly original or valuable. There's no gold in that hill. The value of an idea comes from the skill with which it's developed. The sheer, overwhelming number of titles available on Amazon should indicate that authors certainly don't lack for ideas to develop.
I don't believe in writer's block either: that mental stoppage of creativity. Sure, a writer might reach an obstacle in the story beyond which he cannot figure out how to maneuver his characters around or through. In that case, some distance may be just what the doctor ordered. Let the plot crisis percolate in the subconscious and a solution will likely float to the top. Or delete, delete, delete until you can redirect the character's path and avoid writing him into an oubliette. Another option is to abandon the manuscript. Perhaps it's given you all it can, and now it's time to move on to bigger and better things. Or at least something else.
I do suffer from writing stoppages. Those hiatuses occur because of mental or creative exhaustion. This usually happens after I finish a book and may last up to several weeks. (If I'm really stressed, the hiatus may last years.) My mind feels drained. It needs to rest and recharge. That may include reading books, watching television, spending time with the animals, going shopping or on other excursions, etc. I tend to sleep more during these periods, letting my subconscious play as it dredges up old weirdness and lingering emotions in dreams.
During such a hiatus, I may work a little here and a little there on any one of the two dozen or more manuscripts that I've started and will probably never finish. Some of them were basically brain farts, temporary explosions of limited creativity that eased as soon as I wrote them down. That's one way of knowing whether a story idea is any good. If I can't keep it going, then it wasn't meant to go anywhere, good intentions notwithstanding. Either the story bored me or it quickly tipped beyond even my generous limits of absurdity. Regardless, if it's not worth my time or mental effort to develop, then it's certainly not worth a reader's attention.
That said, I have lots and lots of ideas for stories. Occasionally when I feel restless with the urge to write but I don't quite know what I want to write, I'll go back to those old files and sift through them, hoping to find one or two that spark my interest. I have many from which to choose, too many. Sometimes I'll put in a few paragraphs or a few pages. Sometimes not. I do have several manuscripts started that I want to finish. I don't know that will ever happen, but I have good intentions.
And you know where good intentions lead.