I can. You're in a bar, with a Spanish-speaking friend, waiting for a third person who's over an hour late (not atypical during the four years I was in Spain). You might ask your (present) friend Donde esta? - literally Where is he/she?, with the meaning "I think you probably know wher he/she is and are in a position to tell me". However, you could also ask, in a more rhetorical fashion Donde estara?, using "Future Tense" to mean Where do you think he/she might/could be?. I don't think you can translate it as Where will he/she be without changing the sense dramatically.In particular I cannot think of a single example where the Spanis future cannot be translated by 'will' though I can think of myriad examples where 'will' cannot be translated by the Spanish Future.
In this we are in total agreement. That's why when I teach Spanish I'm very careful not to give the impression that it exclusively refers to future time, regardless of the terminology. Beware, however, as Italian, Spanish, Portuguese, French etc don't use their Future tenses in the same way.Now if you are prepared to admit that the future tense in other languages does not primarily talk about the future I am not going to argue with you.
) that could is the best translation here as you're inviting/asking the listener to speculate on somebody elses whereabouts.
