Interesting, Tec. That idea is sort of a cousin of two other notions you've put forth--one being the ability to rank your wishlist, and the other being the ability to apply a rating to any item. I like both of those ideas because they can be used to generate macro-data about the toys in general. In otherwords, I'd be able to generate lists of the "Most Popular" and "Most Wanted" Transformers.
This new idea of yours reminds me a bit of the "Listmania!" lists on Amazon, which are versatile in that they can have any label the user wants to apply. So if we were to have something like that, you could name the list "Tec's Top 10 Tf's", and then just pick your favs. Someone else could use the same feature to create a list called "10 Transformers that Suck". And then yes, adding it to a tab on the sig would be pretty trivial, and I don't think it would necessarily be giving too much information away or discouraging people from visiting the site, because it would be user-centric custom information, and it might encourage people to sign into the site to make their own lists.
So, it's an interesting idea, and as with most of these ideas, the real challenge would be to come up with a UI that isn't a pain in the arse to implement and/or use. I just had a look at Amazon's Listmania interface, and they do a lot of fancy AJAX magic, which of course appeals to my engineer side. And some of that work might be applicable to the redesign/expansion of the "My Collection" system I want to do.
Let's just say it's on The List, but I wouldn't expect to see it any time REAL soon, there are just too many other must-have meat-and-potatoes features queued-up ahead of it (parts database, restoring the eBay links, upgrading the collection system, etc).
Shmax