A very useful feature in support of backwards-compatible deserializers of protobuf messages is the ability to determine which fields were present in the serialized data but not recognized in the deserialization process.
This would give the consumer at least some insight into data that was present in the serialized data but not available for display (rendering).
Thus it becomes an important (haha important for me) part of a full-featured user interface to the protobuf paradigm (dunno if that helps my case or not).
This would be a feature common to all classes generated from .proto input, so it would likely become a new public method to QProtobufMessage, the base class for all generated message classes.
It could look something like this:
const QList<quint32>& unrecognizedFieldNumbers() const;
I do not know of any other protobuf library implementation that offers such a feature, but I still think it would still be useful.