Touchscreen
With the coming of 8.33 frequencys I would love a touchscreen on the Boeing.... Just like the GTN650/750 for radio selection it is a great improvement on twisting two knobs.
I think the GTN650 moving map is a bit on the small size but as a control head for the G500/600 display is just fine.
On the whole I personaly find Garmin equipment a little less intuitive than the King GPS equipment however as King have lost the plot when it comes to small aircraft there is only one game in town.
I would say that any bit of Garmin aviation equipment is a good place to start as you can move the logic on to there more complicated equipment with a minimum of fuss.