Plugins as code that is integrated with WP are still required to be GPL-compatible. Unless you want to connect to your plugin through some layer of networking a

Plugins as code that is integrated with WP are still required to be GPL-compatible. Unless you want to connect to your plugin through some layer of networking and a GPL licensed proxy, your plugins need to be open as well. Specifically running something as basic as register_activation_hook() means you're linking with WP-provided code.