One thing that limits on the fly strategy changes, or tactical flexibility is the lack of swift, reactive communications. The kind that typing doesn't allow for given the overarching importance of timely actions and reaction and time in general during matches. A person doesn't have time to type "Please send your cruisers to the ping so we can lure the enemy air into a trap with my air" for example, but saying this would be much easier and far more effective, over VOIP.
Obviously it would require an easy to mute functionality. Having the ability to turn off particularly annoying people is a must and perfectly reasonable.
Of course FAF already has Teamspeak channels, but it would be great if somehow teams could automatically be placed within a teamspeak channel in their own teams ready to go from match start up. If that is even possible.
In general though it would be nice to see a more automated and integrated VOIP system as I think it would dramatically increase players abilities to communicate and thus work together more effectively and allow for more interesting strategies and flexibility.