altimeter and vsi
It would be nice ( read very highly desirable ) to have altimeter and VSI based on the pressure sensor in the android / ios device combined with a setable QNH and not only based on GPS.
Are you sure? I recall having read on a few aviation forums that experiments with these pressure sensors turn out to be quite unreliable. This is "scratched" in the back of my mind for a long time already, so we never considered it to be honest. Maybe its time to revise that stand and try to do some experiments.
First of all don't forget the it will never replace certified altimeters and VSI's already available in the cockpit of many aircraft independent if it is based on gps or pressure switch. Many apps have the option to choose either pressure or gps based.
I can imagine unreliabilty in the case of a pressure difference between inside and outside the aircraft. Furthermore the accuracy depends on relation between pressure and altitude, influenced by the outside temperature. Both parameters are not measured and can not be used for compensation.
By coincidende the aircraft I'm flying, open cockpit and hot air balloons, don't have these problems.
Apps I'm using ( mainly in hot air balloon ) for altitude and VSI, beside EasyVFR have been proved to be very stable. Of course you can wonder how it working under different circumstances.
My approach would be : Give the user an option to choose and build up some experiences.