As noted above, you are not going to find actual flight information and pricing loaded into the GDS until late this year. Consequently, you are way too early to be on this decision.
One thing you might want to start to consider is using one of the many "free stopover" programs that are available. Since you will be connecting along the way anyhow, why not add in some additional vacation locations.
Depending on the airline, you can get everything from a stopover at no increase in airfare, through tours and discounts, to free hotel nights. This gives you some excellent opportunities to leverage one round-trip into additional places to enjoy.
European/MEA cities where stopovers are being offered include Istanbul, Lisbon, Dubai, Abu Dhabi, Doha, Reykjavik, Helsinki, Zurich, Muscat, Dublin, Madrid, Amsterdam and Paris.
Personally, I'd be looking at the Istanbul program from Turkish. TK offers some very attractive pricing for their TATL flights, along with hotel and tour benefits. Plus, they offer non-stops from DTW to IST, which means you would be on one-stop itineraries for both Athens and Venice.
But in all cases, just do some preliminary considerations -- you are way to far out for anything else.