Thinking about the 'extra' keys, they don't really make a great deal of sense in this context and I agree would be better handled by BSC or TSC schema messages. Several of the rerences I found to them in any case indicated that they were 'non-standard'.
On Air pressure
AirPressureI=(Air Pressure Indication Rising/Steady/Falling)
Is 'Dropping' the same as "Falling Quickly" ? ie Rising/Steady/Falling/Dropping
I don't agree about retaining the original AirPressure, it would make the enhanced (version 2) schema ambiguous, and perpertuate an existing error.
If a vendor was to adopt the enhanced schema with/because of the extra features, it would be trivial to modify the message to add a P or an H to the key.
Similarly if a user was to use the enhanced schema when scripting/configure their application this would be a small change for them to make particularly taking into account the extra functionality that could be provided
There are parallels in this discussion of AirPressure, with the use of rain= key in the existing implementations which I thought we had resolved.
Is a Light= key appropriate to this schema? If so what are the engineering units and/or range?
I understand your concerns about message/packet size particularly on smaller devices. I have tried sending the full message on my network here with no adverse effects so far. The News.report messages from James mi4 News app can be somewhat larger. I will reinstall a Netiom and monitor an Arduino over the next week or so.
I wont be able to do much on this until next weekend now, but I propose collating and finalising the updated schema then so the wiki can be updated. Obviously any other contributions welcome.