In what direction? Towards an overarching theme? You've got your light and your dark, and the dark side knows to keep the two worlds apart, and the light side wants to merge them. Do you have a reason that the dark queen hasn't told the light king why they shouldn't merge, or a reason that he doesn't know that merging the two worlds would destroy them? Why is he bent on merging them?
It's hard in cases like this where you may have many people with different ideas about how the story should go. Ideally, you would provide whoever is playing the king/queen (if not yourself) the answer to those questions, as well as "at X point in the future, you will decide to do this thing" to further the plot in the direction you want it to go.
Maybe the queen finds out about what the king is doing, and instead of negotiating, or hiding the spellbooks, she thinks that he knows what he's doing. Maybe she decides to wage war on him in retaliation? Or she's more benevolent, and asks for some kind of truce. What if the people who cross over after giving up one of their senses/magics learns the truth? Is there a veil between that takes their memories once they cross back to their own side? What if they want to stay on the other side, regardless of what they've given up? Will it drain their powers or life force?
Or is there a third, unknown force that wants the worlds to be destroyed, and he/she/it is manipulating both sides? Would the king or queen learn this? how do they combat the third party?
I'm just throwing out ideas here, because this is how I brainstorm for my own stories. Maybe something will spark a thought. If not feel free to ignore my rambling :)