"Having to learn a new system every time you pick up a new role-playing game is like having to learn an entirely new operating system every time you buy a computer."
My strong suggestions for anyone considering creating RPGs for a living are:
1. Don't quit your day job yet (unless you can afford it, and maybe not even then) and
2. Start out by designing a scenario or other supplement for an existing RPG like D&D/d20, GURPS, White Wolf/World of Darkness, or Champions/Hero System.
After all, if you can't sell your scenarios or game-world for a system that people already know and enjoy, what makes you think you can sell them for a system they will have to learn from scratch?
It's perfectly reasonable to add new rules, or change existing rules, to achieve your vision within the basic framework of the games that I listed. For example, there are horror supplements for D&D/d20 that have new rules for fear, insanity, and taint!