Doesn't BA still own a 25% stake in Qantas? A merger there would not be all that surprising. I think SQ also owns a stake in QF though, so that would be something they'd have to work out.
I don't see any problem with BA acquiring IB, QF and other, smaller European carriers to become a "mega carrier". In reality they will have to do this just to survive, since LH is on a buying spree (LX, BD and perhaps OS next?)
As for impact on AA, there will be very little in the short term. As we all know, rules governing foreign ownership of US carriers are extremely strict.