I don't know for sure. It could be the king himself as a main character, of course. And perhaps the person to whom he makes the proposal, whether it's a princess, a noblewoman or a commoner. There might also be advisers to the king who play important roles in the story.
The main characters in the 'king's proposal novel' are likely to include the king who makes the proposal. If the story has a romantic element, the love interest of the king would be a main character too. There may also be antagonists, like those who fear the change that the king's proposal might bring. For instance, if the king is proposing a new tax system to fund a war, the wealthy merchants who would be affected could be antagonists. Additionally, there might be a wise old man or woman in the court who acts as a mentor or a voice of reason throughout the story.
Since we don't have more details about the 'king's proposal novel', it's difficult to name the exact main characters. However, if it's a political - themed novel, there could be rival kings or lords who are affected by the king's proposal. On the other hand, if it has a more romantic slant, there could be friends or family members of the person being proposed to who have their own opinions and influence the story.