Look,
I get that Affinity is a small team, but prioritizing a plugin API offloads the work of adding features/formats that people want to the public for FREE. The amount of control we could have over affinity tools would make it worth wild to switch, both Photoshop & paint.net have huge community backed plugins that extend the tools well beyond their default capabilities.
Start small, file formats, expose the canvas, etc. Then eventually you can allow plugins to extend the UI beyond simple list items.
Affinity is a great product, but it could be amazing with a dedicated plugin system.
I can even offer to consult how to integrate it cross platform.