As far is avionics interface in cockpit goes, I would suggest that an intelligent mix of tones and voice warnings must be considered while deliberating on each 'Phase' of flight likely to be undertaken- i.e hover, forward flight (airplane mode), rearward flight (possible ?), hover turns, glide/autorotation and of course vertical pick up and sit downs. While keeping the certification requirements in focus, the pilot-machine interface will finally depend on the missions envisaged and the cockpit (crew) load while undertaking these acts. Thus, consider each phase of flight separately for all the missions listed out by the user agency. Prioritise the parameters through a table and then select the best feasible option for pilot interface while allowing him to focus his eyes at his priority without jeopardizing flight safety/ life of the expensive components. As an engineer you can only optimise the technology for your machine however, making use of all the sensory organs available with the crew (ALL of them) will provide edge over the contemporary. Please remember that even the flight parameters need to prioritised for every phase of flight and using HUD/HMD may not be bad idea. So the voice warnings, tones, tactile cues must be employed together while providing our pilot with only the information necessary at THAT MOMENT.
Hope it helps !!!!!