How and if to advise editor about suitable referees for my paper I am writing a paper where the proofs involve more than one field of mathematics. Call them fields X and Y for simplification. This is not very unusual, but I think it may confuse the editor in the following sense:

There are several motivations for the paper coming from both fields X and Y. However, all technically hard parts in the proofs belong to field X.

So, in my opinion, it would be best to get at least 2 referees, at least one of them from field X (but another one from field Y would be great, at least to give comments regarding the motivation).

While I am not the first one to write about the problem my paper attacks, it is still a rather new problem, and an editor possibly will not have the right "feeling" about what kind of expertise is required for working on this problem.

Even more confusing: Previous work on this problem has been done through field Z, which is certainly not my approach (people from field Z will have to struggle quite a bit with the technical parts of my paper, as I had to struggle with reading their work).

My contribution in this paper is "generalization, theory building, solving some open problems stated by others, stating new open problems". But there is another contribution - I give new motivation which applies also to previous work (not done by me), but went unnoticed.

I would like to communicate to the editor the information regarding what kind of mathematicians I think should review my paper (when I submit it). However, I am afraid it may come out as very rude: Not only I am telling him how to do his work, I am telling him how to judge my own work! Yet, if I were the editor (I am young and not an editor anywhere), I think I would appreciate the information that "this is how the author thinks I should do my work".

Should I communicate any of this information to the editor? What part of it? If coming out as rude was not be an issue at all, I would send a list of possible reviewers explaining why they are good for the job in my opinion.
This certainly sounds like it could be communicated through a cover letter. This does not guarantee that your comments are actually being read by the associate editor, though - at least in my field, cover letters are uncommon enough for journal papers that I typically don't even check if there is one if I get a paper to handle. However, it is unclear to me what practical other ways you have to communicate with a handling editor where you don't know yet who it will be.

That said, you never have a guarantee that the editor will take your input on reviewers into account. Personally, I tend to be fairly sceptical about "suggested reviewers" or anything that approaches this - maybe I am too suspicious, but whenever authors suggest a reviewer, especially when not explicitly asked to do so, I immediately assume that this reviewer will be in some way positively biased, even if no conflict is obvious from the outside. 

So my summary is - sure, (briefly) explain your reasoning in the cover letter, but don't get your hopes up too much that this will change substantially how your manuscript will be handled.