Next step? Well, what have you got? If you already have an idea about an interesting new combat system, then you need to think about how that system can be applied in a game, and actually make it real.
It seems that you start with gameplay. I assume that you've already planned the combat system and how the game will be played in general. Once you've determined the gameplay, think about the technology. Will the gameplay require a 3D world? Or will a 2D environment be sufficient? Think about the control, camera, and other stuff the players will interact with the world with. Then the story. You can't expect the players to sit in front of your game for 50 hours and have fun with it without a story. Fortunately, an RPG can tell a much more complex and in-depth story than other gameplay types like RTS or Arcade Shooter, so the advantage is yours.
Uh... sorry. There are many things, and my memory isn't so good. Gameplay, technology, story. Just think about those three first. That's a lot of stuff already. Think about how they can match with each other.
Once you finish the design, explain the game design to your friends. You can say, "I have an idea for a 2D Diablo-like RPG for PC, and there's this new combat system that will be very fun to play. The game takes place in a kingdom called... bla bla bla" The point is to make your friends understand what will the game be like. They need to know what they're going to make. The programmers then can decide what engine they need to use to accomplish the needs of the game design, the artists can start making stuff according to the engine (an isometric game will need different artworks from a side-scroller), and the writer can start thinking deeper about the story, like the quests, NPC, etc.