Depending on the franchise, you can find many different kinds of collectors. While the most common is perhaps the type of fan who will collect anything they can get their hands on with the property's logo on it, there are others who pick and choose what they want to share their living space with. And then, beyond those, you have people who might focus on a particular aspect of collecting: all of the manga from a series, or all of the model kits and action figures made available. And then there are those who have an affinity for animation model sheets. For those unfamiliar, Protodude explains:
Model sheets are sort of a middle ground item, common on a number of auction sites. However, Upon A Star sheets are quite a sight. As you can see, the sheets boast sketches of the show's characters; animators would use these materials as a reference when animating the show. The depiction of characters in a number of poses and gestures helps the designers to standardize the continuity of a character's appearance one scene to the next.
Of course, whether that is your preference, or you just want anything affiliated with Mega Man, there is a pretty sweet deal going on over at Yahoo! Auctions Japan. For a starting bid of 100 Yen (about $1.18 USD), you can own 20 model sheets "in great condition" from the once long-lost Rockman/Mega Man anime, Upon a Star. Of course, with Rockman Corner (and now us) reporting this, the bid won't likely stay that low for long.
As for the sheets themselves, you can check out some of them from the auction below.
I can't tell which I like better... sad Wily or happy Wily. Except happy Wily is bad and sad Wily is good. I'm confused now.