A few chapter ideas would be. Valerie the Fairy gym leader from X and Y since she had a doll house gym. Sabrina is always good low hanging fruit. Besides gym leaders maybe a jelous girl shrinks a boy or two and just leaves them to fend for themselves in a forest. Or if a tiny got sent back in time by Celebi before the whole shrinking thing occured and either is the reason that this starts or just what happens if someone finds him when it is not a normal thing yet.