LH announced before 9/11 that they were going to begin IAD-TXL service but it never got off the ground, IIRC.
If LH, with significant feed from UA at IAD and from its own flyers at TXL, didn't think (and still doesn't think) it can make that route work, I'm not sure DL's plan will. And, if a single carrier in the market is questionable, I'm not sure expecting a second carrier to follow is a viable expectation.
And, for now, AA only serves FRA in Germany, and then from ORD and DFW. Their second likely city would be MUC, not FRA. And to expect JFK-TXL when they don't even service JFK-FRA (they couldn't make that work!)... again, not a viable expectation.