I think the previous Berlin service was to SXF, although Aer Lingus operate to TXL from Dublin nowadays.
I suspect expansion to hubs would work better to those with long-haul connections. Nowadays, most non-business travellers seem to be happy to take the overland trip up to Dublin if there's a direct flight from there, instead of connecting in LHR, AMS or CDG.
With that in mind, I don't see the Air Berlin hub in TXL or the Vueling hub in BCN attracting many passengers who are looking to connect.
MAD is a different possibility altogether, with plenty of good connections available from there.
Also, remember that if KLM do arrive, they take a lot of the connecting pax and it's a limited pool.
TXL move is nothing more than operational. If you want to offer a good hub service then increasing DUS to connect with AB is most logical, EI probally won't be in OW for a long time. Thing with MAD is I don't think it will that useful for many as it's heavily S America focused where as AMS/LHR East and West where more demand is.