I am probably late to the game, that’s what comes to my mind:
When starting to use XP 11 I was amazed to see how well the out-of-the-box GNS 530 is working. There’s a lot more functionality than with the standard ESP GNS. You can use a stored flightplan and a direct-to without losing any points. There is a pretty well G1000 out of the box, too. They both have SIDs and STARs selectable
Their UI has a resume last flight button which I find practical to use. The new flight button also brings up the last aircraft and selected airport/airport near last position, which I like, compared to the default flight in ESP.
Possibility to set up different profiles for using the joysticks/controllers e.g. prop vs jet etc.
Possibility to select/ deselect the use of sloped runways.
Saving the flight.
The platform should take advantage of modern multicore CPUs and powerful GPUs.
Reasonably updated navigation and variation data and airports.