Hi,
I like a lot of these ideas, some of them are pretty creative! However, I feel like "Being born with them" is a cliche, as a lot of writers have used that and a trigger that activates their powers. A mutation would be a little weird, but writing is about imagination so that would be cool!
"Interaction with a strange plant" would be interesting. Would the characters visit a person's arboretum? Out of all of these, this one is probably my favorite.
I have a suggestion, though I'm not sure if it's as great as these! A significant family member of your main character could be dying, and while lying in the hospital bed, clasped the main character's hand. From that sudden motion a secret hidden power in which the family member had been hiding for many years is passed on, to the main character. (Little confusing!) I hope you like this idea, but your plant one sounds really peculiar.