I’d like to suggest an idea that seems to apply to the majority of writers. That is that most of them do not in fact suffer from a lack of inspiration the way they think they do. The majority of writers of my acquaintance suffer, if anything, from the opposite. They have too many ideas.
How can you have too many ideas? It happens when they start to be a distraction. You get excited about potential new novels when you’re partway through an old one. You add in characters and situations that don’t really belong, because you have to use them somewhere. It can even lead to total paralysis as a writer, because you can’t focus on one thing long enough to get it done well. It’s one of the things behind my occasional bouts of deletion (which seem to get a surprisingly strong response from some of you. I may have to explain at some point why I feel it is fine to delete work).
There are solutions, of course. Rigorous self-discipline and an iron will are always helpful, though annoyingly hard to locate when you actually need them, rather like car keys. Writing ideas down can also help, since that can at least reassure you that you can safely come back to them later. Expressing them in brief formats, such as short stories or poetry, is another solution.
One that I quite like though is that point where you create a kind of literary gumbo. That is to say that you just throw everything in and see what happens. The results can be surprising, and occasionally spectacular, because you get combinations of ideas beyond the well worn ones. Of course, the moment you do that you can no longer rely on your well defined structures and tropes to get through your novel, but that isn’t necessarily a bad thing.
Some tactics for getting ideas in include expressing them in sub plots, bringing them out in minor characters, making passing references to them without ever really exploring them, or showing them in miniature. Alternatively, just write the next thing to come into your head, however weird. It worked for Douglas Adams, mostly.
So why not try it? Stop separating out all your different ideas. Instead, find ways to put them into whatever you’re already writing. The results can be… interesting.