“GPS altitude is the only way to make anticollision work, because one cannot rely on the pilot setting (or even having) the right QNH.”
That is not the future of collision avoidance kit. The future of collision avoidance kit will be FL, it is almost a universal standard, excluding FLARM. “Being different” is only an issue if the manufacturers cannot reliably convert from one to the other. Two manufacturers are launching kit that allegedly does this.
I have configured my EFIS/GPS system and have chosen not to display GPS altitude (there are 100’s of bits of info you can choose from). As it has an altimeter built in, it may be possible to get it to alert if the two are out by more than a specified amount. If I get time I may have a play.
Rod1