While my project will focus on my own characters, I have thought of a way to include other characters without messing up the 'story'.
My character Aspect 3 has the ability to shapeshift into other characters as long as she knows what they look like. If there's a character from a particular series that you like, she can morph into them. 
The morphs will be part of Aspect 3's dialogue tree. You will be able to pick a character then be able to ask her to do certain things as that character. (Note: I have yet to fully implement this mechanic.)
The attached image is an example of her doing a partial morph.