@Dfred Geany is a _small_ project, the workflow tends to be left to the contributors, not all of whom are professional programmers following a formal requirements/design/code process (and even the professionals want a break from it from time to time), and many changes just turn up as PRs so the robust discussion is post-coding much to the proposer's chagrin. @codebrainz use of the mailing list for a proposal was laudable, but that particular one didn't get enough traction, but did save him wasting time coding.
Of course Gmane's searchable mailing list being down doesn't help make it discoverable either.
The way plugins currently tap into Geany features is for Geany to emit a signal (such as file-open and file-close) and the plugins can connect to those. It depends on the functionality involved whether the plugin adds to the Geany functionality or replaces it, most currently add functionality, but there is no reason that they cannot replace it in your case.