Okay so, my boyfriend and I just came up with a new idea:
I've had the idea for a while that the aliens are magic users and they abduct humans (and other alien creatures) to test some kind of parasyte or implant on them, which alters their genetics to make them part alien in some way? Which causes them to have magic powers, too.
Now me and my boyfriend just came up with the idea that, like in D&D, you need material components to cast most spells. But, in D&D this can be replaced by a spellcasting focus (Like a wand or staff). Our idea was that this parasyte would be Finn's spellcasting focus, so he can cast most spells without needing to use any materials for it. Because the parasyte alters his genetics, the magic gets passed on through DNA, but these witches don't have a parasyte to be their spellcasting focus so they need to prepare their spells using materials and the like (just like witches irl).
This doesn't mean I'll be exactly using the D&D spellcasting system, I'm just going to use it as a base.
The witches in the story are descendants of past cases like Finn, if that wasn't clear already.
It'd be interesting to explore how it's hard for Finn to control his magic powers at first. It's easier for the witches, as they have to prepare their spells in some way first. Finn doesn't have this burden, so it'd be like in Harry Potter where when Harry gets angry at Dudley for yelling at the snake, he makes the glass disappear (But a bit more extreme in Finn's case maybe).
I'd also still need a proper motivation for why the aliens are testing this parasyte on other creatures.