If 747-400s are a possibility, what routes would network use them on? As much as some people would love to see DTW-PHX service with them, probably a pipe dream. Sooo....

Probably not main-hub to large city (non-sky team hub) in Europe. While FRA and MAD are busy places, it's probable better to serve them from hubs with smaller airplanes.

Definitely not hub to hub or big city to big city within the US. "People want freq.".

The market that does work for a -400 is big hub to big Skyteam hub. Routes like JFK-LHR (slot controlled so big airplane is better), DTW/ATL/JFK to AMS or CDG would feed each other hubs well and keep CASM down. Also would feed Skyteam cargo.

That's my guess if Delta is looking at big airplanes to buy.