This reminds me quite a lot of the discussion about new IMU messages: Update Rep 145 (P II) by SteveMacenski · Pull Request #372 · ros-infrastructure/rep · GitHub . The conclusion there was that it would be beneficial to keep a common message containing accelerations, angular velocities and orientation, but also to create a new message type for each of these. The idea was that many IMUs can publish each of this pieces of information on a different rate, so the individual messages would allow fine-grained control. However, the overall message would keep the simple interface many people are used to (with the limitation that all data come at the same rate). It was proposed to create tooling that would convert between the two types (it should be quite easy).
This approach seems suitable for me even in the GNSS case. Let’s have a common message with most things people often use together, while also providing more detailed messages for each “subtopic”.
Not sure I understand your proposal (or you misunderstand heading-enabled receivers?). The dual antenna receivers can output heading even without having RTK fixed. They actually don’t need RTK at all for heading. The heading is computed from phase differences of the received signal on the two antennas. And the heading is often given to higher precision than integer. Why should there be a separate “heading integer” when there already is the orientation field? I guess this is where the heading and pitch-or-roll should go.