I think maybe that if you had cards that could only be played in mono decks, you'd cripple the Sealed and Draft formats. I think what may be more feasible is to make cards that favour being in mono decks, such as, "This character gains 1 life for each <Resource> you control." Or items with similar effects.
Currently building a Gearsmith/Banker deck. Any suggestions? Feel free to let me know.
Also, I'm a card-making fanatic. Feel free to check out any of the cards I post and post how stupid or cool they are. Constructive criticism works!