Is there a word for "invented words that are a natural extrapolation of etymology"
Solution 1:
It's a derivative:
A derivative is a stem that is formed by combining a root with an affix that adds a component of meaning that is more than just inflectional.
Glossary of Linguistic Terms
Derivation also describes cases which add an affix without it replacing a part of the word, as in electric -> electricity.
The process that makes derivatives (derivation), is mentioned in the Oxford English Dictionary's etymology for prepone (v.2):
Formed within English, by derivation. Etymons: pre-, postpone