Here’s a short (and not complete) list of datas that can’t be added, because they are technically not exposed by garmin:
Garmin does NOT use the SDK that all developers need to use so they can access more data than any developer of a custom watchface by being able to write their watchfaces in a completely native way.
If new data is exposed by garmin one day, I will very probably add it to my watchface as well but currently there should not be missing any available data.