Talking with @medicmsh yesterday about what a Medic deck would look like reminded me that I wrote a post once on LJ on building a personal meditative deck... so I dug it up (from 2004!), and here it is.
When you're making up your own deck, your biggest task is choosing your symbols. And that, I realized after arielstarshadow's comment, is more important than I thought. Because when you do, you are describing your worldview. Just like in any piece of art, you are communicating (usually without intention) what you believe about life and what it means to be human.
But just like in art, I can't precisely tell you what the symbols I chose say about my feelings on those matters. I chose symbols based on situations in life I have observed that I think should be addressed, and solutions to situations that I think are viable; I chose archetypes based on virtues and values I think are important. There are, for instance, cards in my deck for feminine and masculine impulses: that presumes my belief that the two sexes are inherently different, a belief not everyone shares. That I have several cards in the deck devoted to service has to do with my belief that this is a good thing. Even the fact that the deck's not supposed to be predictive, but rather facilitative, is revealing: I care less about telling the future and more about helping you get through or experience Now.
I think that's why these cards work for me in a way that no other deck of cards could... and why I don't plan to sell them. The Balance Cards are an illumination of my values, beliefs and my way of solving problems. When I read from it, I am giving you my very personal take on life and its problems. It's less like I'm reading cards and more like I'm just using them to help organize my thoughts on your behalf.
If you want to create your own deck, these are the questions I'd suggest asking: what would I want out of a reading? What do I believe about life? How do I think problems should be solved? What symbols speak to me? What virtues and values are important to me... and what do I believe is wrong with the universe, and how would I address it?
Then, I think, you will have the beginnings of a personal deck.