[[https://www.mediawiki.org/wiki/Growth/Community_configuration|Community configuration]] allows tweaking various parameters of the existing [[https://www.mediawiki.org/wiki/Help:Growth/Tools/Suggested_edits|suggested edits]] task types, but it doesn't allow creating new task types. This wouldn't make sense for structured tasks, but for template-based ones it would be very useful, especially for non-Wikipedias where the existing task types aren't really applicable (cf. {T284485}), but it would also be valuable for Wikipedia communities willing to experiment.
The code was written with this kind of extensibility in mind so this shouldn't be too hard. What's needed to happen:
[] check that the configuration loader and search system, the `growthtasks` API, the filtering UI and the task card UI all handle new task types (in theory they should but we never tried)
[] support community configuration for the guidance panel
[] support arbitrary task types in Special:EditGrowthConfig
The config page support seems the trickier part - the usual UX for this is one where you can add/remove form rows, but MediaWiki's PHP form framework doesn't support that. Either we drop PHP support and do it on the frontend, or maybe we could provide a button for deleting each task type, an empty field for adding a new task type, and some mechanism for sorting task types within a difficulty level.
We should probably also add an option to disable the fake "Create a new article" task type.
A low-effort version would be not adding any UI but fixing Special:EditGrowthConfig so it doesn't change any field that's not exposed to the user. New task types would be an advanced feature then, only available via direct JSON editing.