This is a short campaign idea. In The Magician King (spoilers coming) there is a character who ends the book as a King of Fillory. Josh is very into pop culture, D&D and all sorts of other nerdy things. I imagine that as a King of Fillory, he would design a castle filled with all sorts of pop culture reference monsters and scenarios to test its invaders. Making your way through to the end you fight him in a battle where he spouts of trite lines from action and fantasy movies and eventually teleports away, leaving some treasure behind to reward you. Being a benevolent king and videogame lover, those who are defeated are not killed. They would merely be captured or returned to the front of the castle to try again.
This scenario idea would be great because the players would get to laugh about the references, metagame using their pop culture knowledge and have a fun time collectively enjoying the things they all love. All of this could happen in a way that would be consistent with the world's design. It could serve as a one-off sidequest, as a location that is frequently returned to to delve deeper into, or as the launching point into a very deep story where things are not all that they seem to be. It's a fun, fill as you want, idea that I really like.
Imagine fighting off Weeping Angels, Daleks, Cave Trolls, Nazgul, a Balrog, Storm Troopers, Death Eaters, Beholders and anything else you can dream off that you've maybe always wanted to fight. I think it'd be awesome.
It's your fantasy's fantasy.