About the only thing I'd want to add is: know why/who you are doing this for.
It wouldn't be a bad idea to think of the game "outside" of your friends.
This also leads into something else you have to prepare for. For vampires / werewolves etc etc there are some people that are huge fan boys that you will never please. Which Vampire lore are you going to use? Are you going to create your own?
If this is just for your friends then who really cares.
I created a Transformers system a LONG time ago that never gets played due to my current group not being into the genre. I blame the movies. The game was based on the cartoon, not that Michael Bay crap.
It might be just me, but I don't always place RPGs and video games on the same shelf. It bugs me as a RPGer when people think of Zelda as an RPG.
My best advice is play test, play test and play test and a saying from the scriptwriting world "Don't be afraid to kill your babies." You may have an awesome idea but you might be the only one that feels that way. Be prepared to take some hits to your ego but just let it slide.