So we have two notifications pages then?
No, I'm not being clear I guess. The subscription plugin just adds the ability for existing notifier to have the ability to be subscribed to. The current page/UI is still used for preferences as well as for the notifications. Basically, it just adds a layer on top of the current system so that a plugin or core feature can add the ability to subscribe to any object (board, topic etc) which's notification is pushed through the current system. It adds no new UI element (the topic subscription one just replaces the URL of the current Notify button).
Basically here's how it goes (topic subscription plugin is an arbitrary example)
A user clicks "Notify" button on a topic -> the request gets sent to the subscription core which registers the ID and type of the subscription.
Another random user posts a reply -> the topic subscription plugin has a new_post callback which calls the subscription core to issue notifications
The subscription core issues notifications passing the given data to every subscribed member -> The topic subscription notifier kicks in and delivers all the notifications
You can install and try, or I can finish the patch in a couple of days (already WIP) and post it to merge.