Ability to create Flag lists? for example to have multiple bookmark lists or friend groups.

Flag is an awesome module and thanks so much for creating it. I currently use it to allow users to bookmark (flag) a content page on my site. This is great, they can click bookmark and then with views I can have it display a list of all content they bookmarked on their user page.

What my users have been asking for though, is the ability to flag the content and add it to custom lists that they make. I was thinking to just create a new module from scratch called bookmark lists, but I'm wondering if it's easier to make a add-on module to flags or add this functionality into the module itself. As far as I know, you can create multiple flags, but those flags can only have one value on or off.

What I was thinking is you have two types of flags, simple ones that are just on or off and then ones that can have various subflags within that flag.

So when a user clicked on the bookmark flag, it would ask them which list (or create a new list) to add that bookmark to. Then users can flag content but assign that content to fully customized lists created by each user. This can be applied to many types of things. Another one I could think of is using flags to FOLLOW or Friend people. I currently use flags for this as well, but again, I can't create groups. They are just a friend. It would be nice if the user could create groups like (friends, family, coworkers) and then when they clicked the friend flag it would allow them to assign to a group.

I'm not sure this is best to be added into the flag module itself, or it should be separate module that interacts with the flag module or a completely stand alone module. I was thinking of ways I could make it as a standalone module, but I rather have it included in the core flag module or be a add-on type of module. I just have NO idea how to begin making a module as a add-on to another. But I would be willing to put in time and effort to help.