Please help with my plot development!

I've had this idea for at least two years, and I love it, but I don't know how to develop it! Here is the basic idea,

There are four types of humans in this world, Mages, Sorcerers, Witches, and true humans with no magical ability what so ever. There are also magical creatures. My main character (Eisley) is the child of a magical creature and mage who never learned to use her magical ability. Her father who is a type of creature able to shape shift, disappeared before she was born. Two traveling mages (Oles and Wren) come through town and her mother asks if they're willing to take on an apprentice. After registering her as their apprentice they continue to the capital where their home is and start teaching Eisley magic.

But this is where it is really underdeveloped, as Wren and Oles are mages of the capital, they have official meetings to go to regarding the magical world. At one of these meetings, it's discussed that mages across the world have been disappearing in a sort of pattern. After further investigation the mages at the meeting find that it's a message spelled out with the homes of the mages that have gone missing.

The bad guys are Eisley's dad and his lover, a devil. They have found a way to change mages into literal monsters with a certain type of drug and are trying to perfect it, but they wanna have a little fun while doing it which is why there is a message in the homes.

It's really underdeveloped but I like the ideas that I have, even though some need changing. Do you have any ideas for leading up t the discovery of Eisley's dad being the kidnapper? Do you have any ideas at all that will make my story actually sound good? What about questions? I'm willing to do almost anything to get this going. Oh and also the title is, Paperstars

I should add that this apprenticeship was accepted on accident. Eisley's mother asked Oles while he was intoxicated and Wren doesn't want to get him out of it. :-)

Maybe instead of spelling the message out with the homes for fun, make the message on purpose. Maybe they are trying to lure someone out. For example, Eisley. They make sure that her apprenticeship is accepted "by accident" so that she'll be privy to the details about the disappearing mages and they'll be able to send a message only she'll understand.

:D Thank you! That's a really cool idea! I'll make sure to consider it and possible outcomes when developing my plot! :3

Maybe Eisley's father only had her so that he could experiment with her genes to make a monster or super soldier or the drug. And so when her mother found out about it, she ran away with her unborn kid. And in an attempt to keep Eisley safe she tries to pretend hat both of them are human. Maybe its Eisley that asks Wren and Oles to take her as their apprentice.
Something that could also give your story some depth is finding out what the characters motivations are. Like, Eisley wants to find and stop her father because maybe its his fault that her mothers sick/dying/in a horrible mental state or she feels like its part of her responsibility. And the motivations of Wren, Oles, and Eisley conflict sometimes and they have arguments and they get closer.

I also think it would be really cool if it was a story that took place over a long time, so that the reader could see Eisley grow and learn magic (cause I'm assuming it takes a while to really learn magic and use it well) and develop in her personality and friendships until she figures out what she really wants aside from stopping her father.

Maybe there's an ultimate sacrifice from someone she cares about in the end, so that she has a chance to defeat her dad. Maybe none of them survive or she has to sacrifice something she loves in order to win, like her future as a mage.

Maybe there's a tense relationship between the different magical races. Or Wren is a sorcerer pretending to be a mage cause he's in hiding from someone. And Oles is highly respected in the Mage community and a part of his character development is losing some of his super traditional views about the separation of races, especially after he figures out Wren's secret and they have to use their different forms of magic to survive something.

Maybe Eisley is looked down upon by other mages because she's so unskilled (Because her mom didn't teach her magic) and is still Oles' apprentice. Maybe Wren and Oles don't know that she's part shapeshifter and she tries to keep it hidden from them because she thinks they'll think lesser of her for it.