Compared to some of my other feature requests, I think this is 'easy'
.
The concept is simple - in your profile you can create as many 'collection groups or categories' as well as 'wishlist groups or categories'. When adding an item to your collection, you have the option of selecting a group/category you created. For instance, you can break up your collection, by saying some of it is with your child. You can also break it up by saying some figures are located in this area of your house or this box - however you want.
The same with your wishlist. You can create your "Must Have", your "Holy Grail", your "only on Super-Sale" wishlists.
The Default category can be renamed to make it easier when the feature is implemented.
When a Group/Category is deleted, all items in there would be moved to the default.
When browsing your collection or wishlist, you have the option of selecting your category or group.
I hope I'm not the only one who thinks this is a good idea... because I would really benefit from it (especially since I don't really use the wishlist feature for this lacking functionality I need).