To answer your question:
I find that I constantly struggle with the law of unintended consequences when I design games. Unless the skeleton of the design is extremely well structured, the more rules that there are, the more likely it will be that a perverse incentive structure will emerge in permutations of play that I did not anticipate.
Court of the Medici was originally a 2-4 player game. However, during playtesting, I was unable to find a solution to a problem of intentional, or unintentional, king-making. When someone makes a mistake in a two player game, then they suffer the consequences of their mistake. They lose and justice is preserved. However, when a mistake by one player can lead a third player to lose despite all their carefully laid plans, then justice is lacking for the third player. The game becomes not fun. My attempts to rectify this problem only created even worse problems, so the number of players was reduced to 2. Despite the fact that it had already been sold to the publisher as more than that!
I also toyed with the idea of giving each different number a special power, not just the minister, jester and lady-in-waiting. Since the value of the cards for scoring is inversely related to the value of cards for play (ie. 10s are as valuable as they are vulnerable), I realized that every card could have a special function without unbalancing the game too much. However, this idea never developed very far on account of two main reasons: (1) every additional power would exponentially increase rules clarifications for card interaction (2) the game is mentally taxing as is – more special powers would add an unnecessary, and unwanted, additional level of complexity. The three powers that do exist are very simple – and decently intuitive. In the end, they were chosen in order to balance each other and solve problems associated with game play that emerged in playtesting. Once those problems were solved, then it became unnecessary to add any more.
Your variant is interesting though. Insofar as it changes the fundamental mechanic of the game, it may allow for many more additional rules without resulting in too much mental complexity, king-making or rules clarifications. If it can do this, then it has promise for solving a problem I could not (multiple players) and integrating an element that I personally find fascinating (collectability).
