Flying from Athens to Chicago

Your question is not clear. Are your options?

(1) Fly from ATH to a European airport, then Transfer to a Nonstop from Eurpe to Chicago, or
(2) Fly from ATH nonstop across Atlantic to some eastern US city, then transfer to a Flight to Chicago
--- unless there is some terrific price advantage, I cannot imagine booking a flight with TWO stops, such as:
(3) Fly from ATH to European Airport, Then Transfer to transAtlantic flight to US East coast, then transfer AGAIN to reach Chicago.

My nonstop fllights from ATH to Philly are about 12 hours ... it seems like less because of 7 hr clock change; you take off from ATH at 9 or 10 AM, ... land about 4:30 pm philly time... but in your body it's 11:30 pm. Nonstop from ATH to East Coast options in Nov, would probably be limited to NY, Philly, Atlanta, Newark, maybe Boston?(not sure in Nov). Where do u want to visit?

If it were possible to stop over in a European city, would be nice if one could pick a smaller international airport: Brussels? Dublin? but it would need to be one that offered a nonstop to Chicago.

You'll get best help if you supply more specifics.

Lisbon , Barcelona and Madrid all have direct flights to Chicago. By Nov., weather should be pleasantly cooler.

If I understand, yes.

Realize that direct from Athens to Chicago is the shortest time in the air. Takeoffs and landings at intermediate airlines adds lots more time. Plus, you get to do more immigration, customs and security at another airport. For me, direct flights may be a little more money than connections and I will usually choose direct due to all the hassles and risk of delays of connections.

If you want to chop your flight up into shorter segments, then fly to London, Dublin or Amsterdam. They have flights that take about 8 1/2 hours to Chicago. If somewhere warmer is wanted, then agree with joe32F on Lisbon.

If you simply want another city, the world is your oyster...

Using Google Flights, one way, from Athens, To "Europe", Explore, and then on the map page filtering for non stop, there were quite a few options. Besides Lisbon and Barcelona, I see Vienna, Rome, Zurich, Berlin, and London, among others. All you need to do is to see which of those cities have non stop flights to Chicago. Pick the one you like best.

All the return flights are not nonstop but 1-2 stops and take 16 + hours.

Lufthansa has a couple of one stop options that take 13.5 hours, (not 16+.), if that makes a difference to you.

Perhaps I misunderstand the OP: United has a nonstop for 11:40 (which usually would be less if things were normal). Right now, UA 423 departs Athens at 12:35 PM every day.

AarEm, are you sure that non stop isn't seasonal? I can't seem to find a UA nonstop in November.

It is my experience most of the non stop flights from/back to Chicago from Europe end Oct 31st
We have had to connect twice in the past few years for our return in Nov when we flew out nonstop in Oct.
This time we are leaving and coming back in Oct so we can fly direct.

If possible I would avoid London as a connection.
If you can connect through Dublin you will go through emigration there & not have to when you land in the US which is a big plus

I stand enlightened about Athens to Chicago, sorry. I would also avoid London as a stopover (simply for the airport). Or CDG, but I have myself never flown through there. For me, MUC and FRA or VIE and even ZRH have been good connections. I have flown Star Alliance for the past two decades only. I used to fly whatever alliance NWA was on before that.