Why G3 and N3 successors remained undefined

How much is known about planned follow-on classes to the G3 and N3 designs?

Very little, because Britain intended to learn from the ships in service before designing their successors rather than plan several generations ahead.

Unlike American and Japanese programs that needed long-range legislative or political authorization, British practice expected to build G3, then N3, and use experience from both to shape a later class. Treasury bargaining occurred nearer the construction decision, so there was little need for a detailed paper successor. A later design might have explored higher speed and higher-pressure boilers.