Implications of creating a plugin that adds functionality to other developer’s plugin?

If a version of the plugin is available under the GPL then you risk nothing by creating your work and releasing it under the same GPL.

If there’s a reason you don’t want to publish your code under the GPL, then it gets more complicated.

I referred to this which gives clarity about what is a ‘derived’ work, and I believe if you use the datastructure or code from another plugin and rely on it for your code to work (e.g. you call a function from that code) then it’s reasonable that your project is ‘derived’ and therefore subject to GPL.

But it’s not really worth arguing the point if you’re happy to publish it under the GPL anyway – then you’re covered either way.