How to respond to an e-mail asking me to suggest a doctoral research topic?
I would suggest, unfortunately, that you refrain from giving students research topics. That is, unless you are able to continue to guide them through their degrees remotely. I doubt that you are able to do that, hence my recommendation. 

It would be different, of course, if the request were coming from the advisor of such a student. In that case, you can examine what it might be that the advisor is most capable of doing for the student. 

As a positive response to the student, and a general one that requires minimal tailoring in each case, is to ask the student to connect with a local advisor and, if the advisor is unable to come up with something, to have that advisor contact you. 

But I think that local advice will be needed for nearly every student. In the very rare exceptions, the initial letter will probably make that clear.