Replace the simplified helicopter flight model with a more realistic one (preferably leaving using default FM as an option).
Integrating support via RotorLib library (as was done in ToH) would seem like the most straightforward solution. However, other options might exist, too (for example JSBSim).
Last known DEV comment on the subject:
Don't get your hopes up. The awesome Arma 3 from 2012 is in the trash for some reason (Maybe a DLC in 5 years :P)
I agree there are some issues, but calling it trash is too much IMHO. If BiS is affraid of how much time team would have to spend tuning physics for individual helicopers (I remember it being a big issue with TOH), I'd say all we need is an ability to make our own settings.