Unpopular Opinion: In Game Voice Chat
Devs have stated multiple times, on multiple forum posts and on multiple streams, that the goal is to "bring solo players 'skill' up to the same level as SWF + VoIP".
So just implement in-game voice that only survivors can hear and get it over with. We know this has already been thought about in house, so just bite the bullet and do it. It's the only way this plan can possibly succeed.