Learn Spanish (Latin America): ¿Quién será ese desconocido en la plaza?

In Spanish, the future tense is often used to express a guess, wonder, or probability about the present. By asking ¿Quién será...?, you are literally saying "Who will he be?", but it means "I wonder who he is?" or "Who could he be?" right at this very moment.

The word desconocido looks like a past tense verb. How does it mean "stranger"?

Desconocido is the past participle of the verb desconocer (to not know). While it is often used as an adjective meaning "unknown", here it acts directly as a noun meaning "stranger" or "unknown person".

Why use ese instead of aquel for "that" stranger?

Both mean "that", but ese is used for things somewhat close to you or the person you are talking to, while aquel is used for things far away from both of you. Using ese suggests the stranger is in sight and relatively close by in the plaza.

Could I just say ¿Quién es ese desconocido? instead?

Yes, but it changes the tone. ¿Quién es...? is a direct question expecting a factual answer ("Who is that?"). Using the future tense ¿Quién será...? makes it a rhetorical question or a shared musing ("Who could that be?" or "I wonder who that is?").