The Inspired campaign
Alright, so you want to make your campaign using an existing setting, something from a favourite book, show, movie ect. There are several factors to consider carefully.
Will my players get it?
This is one of the first mistakes that can be made. No matter how amazing and in depth your Gundam campaign might be it will fall flat if your players don't like or know about Mecha. Even if they happen to like a different Gundam series than you, it could wind up being a very off key experience for them. If the players aren't familiar with the source material are they willing to take a look? Or does your campaign stand enough on it's own two feet that no prior knowledge is needed. Generally the further from traditional a setting is the harder it will be for a player to go in blind and maintain the tone you're going for.
Will it make a good campaign?
This is another really common question to ask yourself. We've all been there, we watch or read something, we get super excited, we want more and we want to play in the world we saw before us. We want to tell stories as good and exciting as what we just witnessed. But not all stories make for good role-playing games. Maybe you love the classic spaghetti westerns. For a Few Dollars More, The Good the Gad and the Ugly, Fistful of Dollars, these are classic tales of a lone badass gunslinger come to enact a form of bloody justice. But does it convert into a party of four to six players? Not easily. Consider that stories are told very differently in role-playing games and think carefully about if the spirit of what you want to emulate can survive the transition.
Assuming that the above questions are answered affirmatively there are a few more considerations to make.
What game are we playing?
This topic happens to be one I wrote an article about just last week. But in short it's important to know what game you're playing and why. Along with this you need to consider what rules you're changing, removing, or adding if any. Make sure the roleplaying game rules match the tone you're going for. Which brings us to tone.
What is the tone of my game and how do I keep it?
This is another topic I've brushed on a couple of times in past articles. The important thing is to know the feelings you're trying to evoke and to consider how important they are. If you're running a game set in a combined Pixar setting, you need to consider what happens when the players want to make dirty jokes and kill things. It's not necessarily a bad thing if the tone your players find is different from the original idea, as long as everyone is having fun and you're on the same page and you can roll with the punches. Some game concepts though are better served with a specific tone, and in that case it's important to get the players invested into the game, the more immersed they are the more they will feel compelled to suit the mood around them.
How close is the original story?
This can be another crucial choice. Let's take for example a Lord of the Rings game where the players play as Frodo, Gimli, Legolas, and Aragorn on their famous quest to destroy the One Ring. Compare that to a game where the players are a group of Dwarves leading another expedition reclaim Moria. And contrast that to a game where the One Ring is instead entrusted to a coalition of the finest warriors from across middle earth. Each will feel like a very different game, and each will involve very different interactions with the original "Cannon" of the franchise. Some games are best served by being in the same setting but only briefly hearing about or being influenced by the main plot, others are most satisfying when the players get to rub elbows with, or even take control of, the famous characters that make the source material great. There is no right or wrong answer as long as everyone is in agreement.
How long will interest last?
This last question is important for planning how long the campaign will be. If your game is based on a movie that just came out, will players stay interested 3 months from now? If your game is based on a videogame, what happens when all the players have beaten it and moved on to other games? Or is your campaign revolving around a continuing series that will keep providing more and more free excitement building? Making your game too long to hold interest, or not long enough to take advantage of excitement can lead to a less satisfying game overall. So consider carefully how long the fires of passion can stay stoked in the hearts of your players.