I think it would be useful to have beatsync controls that would allow midi mapping scripts to specify a deck to sync to - this would allow syncing of samplers to a specific deck. From a few quick tests in 1.11 Beta2, it seems beatsync from a sampler will sync to deck 1 if there's a track with a detected beat on that deck, and deck 2 otherwise. Default behaviour could stay as-is, but there are situations where specific control of what deck to sync to would be useful in scripts - maybe leave the beatsync controls as-is, and just add a set of beatsync_to controls or something.
This feature would also seemingly be useful once the 4 deck features start being used - with only two decks it's obvious that you want to sync to the other deck, but with more than two decks and samplers, it won't always be clear what a user wants to sync to.
I suppose another way to implement this might be to allow scripts to designate any deck (or sampler) as the "BPM master", and then any beatsync commands would sync to that deck. This might also facilitate eventually setting up some kind of timecode output to allow other programs to sync BPM with mixxx.
This will be addressed as part of the master sync work