Along with the software repository, it would be nice if there was a way to document/organize a number of things so everyone could find the information.
For example, I've working on a spreadsheet of the 3 different HAI wireless implementations I know of (GE, Linear and HAI). Not just the devices but the meaning of the bits HAI reports on the PC Access status screen, cross-referencing the GE/Caddx/ITI/UTC/Resolution/Alula part numbers, documenting things HAI failed to implement in the wireless such as tamper and cover. This is a real problem with GE wireless smokes as HAI instantly goes into fire alarm when the smokes are taken down to replace the battery. The correct response is the HAI should report "cover" or "tamper".
Another item of great use is the note someone posted a few months back about how to recover the installer password from the static ram. This is even more important now that Leviton is no longer factory resetting boards.
I think people like @RobinsonNi
could help with repair tips on some parts that he is working on.
Other things are somewhat known but not well documented. The 2 different temperature sensor hardware types. Why some don't work on the ProII expansion panels, etc. The fact that high and low temp alarm settings are forgotten when ever the panel is unplugged and the battery is disconnected or when the panel goes into reset. This is a problem for vacation homes as temperature monitoring is very important in the winter.
Another not well documented issue is when the system time gets reset, all time related programming breaks. For example, the weekly call out to the monitoring service to provide the test signal quits happening as the system doesn't know the date so it never knows when to send the test signal.
Another item is firmware images. A number of us have eprom programmers so we can update our systems. I don't know if its widely known there is a 4.0 chip for the OmniLT. I sent an OmniLT in for repair shortly before they stopped repairing them. It came back with a 4.0 chip (hand written label). The OmniLT is a surprisingly capable board. With its expansion board it supports more hardware zones and outputs than the replacement OmniLTe. 24 in 10 out vs. 16 in and 8 out for the LTe.
Another item, the info on 3D printing the stands for the touchscreens. I had a standard console recessed mount 3D printed for me but have not been able to get the specs used to create it...
I'm sure there are lots of items out there known by others.
Mostly throwing this out to spark some discussion.