This is the summary of a discussion with Bertl on 6.12.2018:

The following interfaces and applications are interesting for potential shields in the future:

• sync/trigger/timecode IN/OUT
• audio
• HDMI/Viewfinder (1080p) Output
• luminance sensor
• Time of Flight (ToF 3D) Sensor - needs research

East and West shields are already different in design as the slots provide different lanes with different speeds so putting an east shield into the west slot and vice versa will not work.

There is the general question how we could design the shields in a way that they can be utilized with the ABCP AND ABDK.
ABCP requires connectors in specific places so they fit enclosure.
Some shields like the debug shield will likely only be interesting to ABDK users.

For the compact the general problem is that in the location where the shields are placed currently its very difficult to add any connectors or interfaces to the enclosure, especially on the west shield side.
Bertl had the idea to create a small PCB that goes from the shield header directly to an FPC. We then design a small PCB for the above mentioned applications that can be placed somewhere else in the camera and is attached with a ribbon cable or FPC. This means any shields designed for the ABCP also work with the ABDK but with the ABDK there is no easy way to attach the shields.