https://en.wikipedia.org/wiki/User:Risker/Risker%27s_checklist_for_content-creation_extensions is a very useful checklist, based on experience and iterations. It should be reviewed and integrated to #WMF-product-development-process.
This might affect several stages, but a clear default candidate is https://www.mediawiki.org/wiki/WMF_product_development_process/Concept