What would be an interesting subject for a Greek tragedy that follows all the rules, with the action taking place in one day, in one place, and...

This is an interesting project. In addition to the three requirements in your inquiry, a “Greek tragedy” must also have a protagonist who falls from a high place, due to his/her tragic flaw, who is exposed before a “chorus” of affected citizens, and whose fall gives the audience a “catharsis.” You will want to begin with a protagonist who falls from a high place, so you could start by deciding what world your story will dramatize – a modern business world (where the protagonist has political or financial power) or a historical period, where a royal figure or a great warrior has earned a high place in his/her community etc. Next, you must choose a circumstance in which action is volatile, the protagonist has a tragic flaw, and the atmosphere is charged with possibilities. Perhaps the most difficult requirements will be the involvement of a “chorus,” and the final catharsis for the audience. Some possibilities, then are:

1. 
   

   One location: A board room of a Fortune 500 company; one day: a decision must be made to buy out another company; the fall comes when the CEO is discovered to have made an unethical compromise.

   
   


2. 
   

   One location: The parade grounds of a might army; one day: the protagonist is to be awarded a Medal of Honor; the fall comes when a fellow officer accuses him of cowardice on the battlefield.

   
   


3. 
   

   One location: A meeting of an organization of authors in a Gentlemen’s club; one day, they are to choose this year’s literary prize recipient; the fall comes when the main contender, the protagonist, is discovered to be a plagiarist.

   
   

These are just some of the possibilities. I hope they prompt some original ideas in your head.