How to publish a paper that does not seem to be within the scope of any journal?
Interdisciplinary papers have this problem a lot. A - now - very influential paper in my field, I was told by the first author, was in limbo for 4 years before it was published, despite the senior author being world-famous. 

Persistence is the key. Try to find someone influential who is knowledgeable and you can convince to help you expanding/adapting/explaining the work better and you could then co-submit with. Or else, you could consider aiming for a general-purpose journal such as PLoS ONE, and whose reviewers are advised to select by novelty and correctness, rather than (subjectively judged) relevance. 

Learning to write for the specific audience of a journal can also help.