I think a great feature that's easy to implement would be to add a variable system.
What I propose I that you can access a menu containing all your variables, main idea being numerical and color variables.
Then in any context e.g. line width, fill color, opacity, etc. you can enter e.g. $var1 / $primaryColor to use this value.
This would make it very easy to make batch changes, especially in affinity designer which is intended to create, allowing fast iteration through art, logos, advertisements etc.
