I spoke with a computer scientist a few weeks who told me that in computer science there is something called a "vision paper", which is a paper that does not contain concrete results, but rather outlines a more general vision of what one either wants to do, or how a field may develop etc. A future vision in any case is presented.
Searching the internet "vision paper" for computer science actually did not return many results - lest so for mathematics.
But maybe there do exist such journals that offer a home for such articles in mathematics and it just happened that I didn't find that. If you know of any, please let me know. The journal doesn't exclusively have to publish vision papers (actually, I think that would be bad, since it would probably be soon filled with all kinds of fantasies regarding how some subfields of mathematics might develop), but should also allow such papers. There are some journals that allow papers containing open problems and conjectures. I am looking for something a bit broader than that, where a vision of a field can be presented that is extrapolated from the current state.
I am interested because I myself am about to write such an article, and I'm not sure where to best post it. On a blog? I'd like to make it citeable and everything that purely online may vanish at some point in time (e.g. the blog providing company may shut down). A paper solely in arXiv? Would be an opportunity, but perhaps lacks in credentials.