I like the idea. Most stories do not include where magic comes from, and your idea of how two different worlds or dimesions with two different laws of physics come together kind of helps that.
What you've done so far is really good, too. Jot down your ideas for now, then start weaving them into a story. In the end, you should have weaved a cloth of an epic!
I should keep going on my story too -_- kinda stuck on it.
Here are some suggestions to further develope the story into detail.
self-question:
Ask yourself questions that your intended audience would think about.
i.e.
Q: "How did Mimi come to the power she has?"
A: "Mimi had volunteered to give herself up for human experience by scientists. They had held her younger brother hostage, she had to give in. During the test of a new chemical that is supposed to 'regenerate the body as fast as it gets hurt', a 'Holder', people of magic conjuring abilities, comes in and raids the place as means to take over or for whatever reason he needed to do it for. Mimi then evolved from the magic combined with the chemicals she had taken. Her arms then began able to change organically. Whenever provoked to anger, her arms will unwilling change until she later gains control of it.
Mimi's brother dies during the raid.... She sought out revenge and traced down all but one scientist (you can use him for whatever later) and the Holder."
I could go on forever, but you get the point
After self-questioning, you want to think of the characteristics of your character.
Jaque might sound like a british panty-in-a-knot bard, for example