I can't stop thinking about a daddy-longlegs, alternate dimension au for Harry and Tom.
Like Harry gets sent to a parallel universe for some reason and decides he might as well change Voldemort's childhood.
He never approaches Tom, but he organizes a caretaker for him along with a house and all monetary his needs. He writes monthly letters to him checking in on his schooling and life in general but that's it.
Meanwhile, Tom tries and tries to find his mysterious benefactor, but Harry's always a step ahead. He becomes obsessed, irritated, infatuated with Harry all along the way while Harry is none the wiser.
The two accidentally meet a few times (Harry's absolutely sweating bullets during these moments) but luckily the secret stays hidden. However as the years continue, Tom gets closer and closer to unravelling the mystery.
After graduating Tom figures it out, but it's too late. Harry's returning to his own world. Tom finds Harry right before he's leaving and says he'll find him no matter how long it takes.
Cut to ten or more years later and Tom finds a way to Harry's world and leaves to reunite with him.
