LH FRA
As much as I'd like to see the return of Lufthansa / a FRA link I doubt it'll happen anytime soon. A more realistic alternative might come in the form of MUC depending on how quickly Eurowings decides to expand the new operation there. MUC is also a lot nicer to transfer through as opposed to FRA.
That said flight times will play a big factor, you'd really need two daily to properly open up connections, so perhaps BMI with a LH codeshare would be more workable.