These smaller features are likely to be overshadowed by the release of the G1000 for default aircraft in 11.10, so I decided to dedicate a blog post to promote the articles I've written - you can find them among all the guides for aircraft developers: http://developer.x-plane.com/docs/aircraft/
Electric and remote gyro systems
Back in April, I flew a Mooney M20J with a KCS55A HSI in it, and realised that it was impossible to model in X-Plane correctly, so I got to work. See the manual for an explanation of this popular HSI/remote gyro system.
I've written a usage guide on the new datarefs and commands that I added, along with some more detailed explanation of all the different gyro systems X-Plane simulates, in this guide for aircraft developers. I also talked about the systems at length in a Youtube live stream earlier this year.
Separate GPSS autopilot mode
This is a feature that many add-on aircraft already simulate to some degree, but by means of more or less reliable plugin trickery. The X-Plane 11 default 737 and 747 are no exception. With X-Plane 11.10, a separate GPS steering mode for the autopilot becomes a standard feature.
The new datarefs and commands are explained in detail here.
Screen-only popup instrument windows
Several people who build home-cockpit setups have asked about removing the bezels from the popup displays, so they can have only the screen of a GNS430/530, FMS or G1000 instrument to put on an external monitor, with a hardware bezel around it. While this can already be achieved through some clever hacking in the Miscellaneous.prf file, we now offer a more straightforward way to do this: The popup and pop-out windows now get their bezel graphics from the library system, so you can override the bezel graphics. How to override the bezel with nothing, if your bezel is made of hardware? Simply supply a 1x1 pixel blank .png as a bezel graphic, and X-Plane will know that you really want no bezel at all. In the case of a bezel-less 430, you'd put a 1x1 pixel png as the "cockpit/radios/GPS FMS/Garmin_430_2d.png" resource of your plane.