Perhaps a sticky containing useful code fragments, like this calibre recipes sticky here:
https://www.mobileread.com/forums/showthread.php?t=70263
The builtin plugin API should be stable and not change in every version just because someone thinks of a new interesting thing you can add to it.
And this allows people to share complicated stuff as well, things that stand less chance of being built into Sigil.
Also means people don't need to wait for a new version of Sigil in order to publish a plugin...