You're approaching this backwards. First, figure out what form of media you think you can work with. Then, figure out what kind of story you think you can tell with that medium.
You can tell just about any kind of story with any kind of medium if you're able to use it well, so being able to work with your medium is your biggest limitation. Are you and your cousin willing to either do all the work involved in making an entire animated series or to hire other people to help you with it? A video game? A comic series? A novel? All of those things require a lot of work, but they take vastly different amounts of work and different kinds of work.
Once you decide which kind of media you can actually make something with, then you decide which one you can tell your story with.
However, if there is one medium that you
really want to work with, yet you feel like you'd be unable to tell this story with it after you've evaluated the amount of work it would take, then start of with a smaller project with that medium instead.
It can be easy to overestimate the amount of work it takes to work with a medium you have no experience with, so you should ideally start as small as possible and work your way up from there. For example, if you want to make a video game, don't start out with your ultimate dream project that would take years of dedicated work to complete; start out with something very simple, like a traditional arcade game, and see where you can go from there.
In other words, if you plan to go big, you have to start small first.
Regardless, if you want to tell that particular story and do it justice, then you'll have to work with the medium you think you can do it in, and which medium that is depends very much on how much work, time, money, and experience you and your cousin have at this point.
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