I had this idea and, like with most of my ideas that I randomly get, I fell in love with it.
Art by chokomoris on Tumblr
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Just so you know, this is also a modern!AU. Like, it's still IRL Minecraft, but more modern.
So, Wilbur's moved out, and Phil let Tommy go with him (or you can have there be another reason as to why Wilbur and Tommy aren't with them), So Phil and Techno decide to use their vast amount of money to buy and move into a really old mansion. It's in good condition, it's got a great forest, and it's really big.
So, after they've gotten settled in, they start to notice weird stuff going on. Items move by themselves, break spontaneously, electronics turn on and off on a whim, cabinets and doors slamming open, that kind of thing. After Techno almost gets hit by a flying object, they decide to look into the situation and come to the conclusion that there's a poltergeist haunting the old mansion. Bonus points if they called Wilbur and he told them, because he totally strikes me as the type to have been really into supernatural stuff.
They perform a summoning ritual, preparing for the worst, only for this tall, hybrid child wearing old-timey clothes to materialize in front of them. He's bawling his eyes out, shaking and apologizing for breaking things and almost hurting Techno. Turns out, he's not hostile, but he either has no control over his powers, or he's just clumsy.
They console him, telling him it's not his fault, and basically adopt him into the family as their new child/little brother.
Just imagine them bringing the other two boys over for dinner so they can introduce him, they're just like, "Hey Will, remember that time you told us we had a poltergeist? Well, you were right, here's you're new brother."