If I had magical engineering powers, what would I build? Seriously?
A TARDIS, duh! Post’s over.
Oh, you wanna know why? Well, think for a minute, will ya? Sure, I could build a replicator to replicate food and other goods. OR I could go to a future where that already exists. Sure, I could build a robot servant. OR I could go to a time period where that’s already mainstream. Sure I could build a super fast quantum computer. OR I could pop into 2236 and try theirs.
I mean, I could also build a space ship… oh wait, I have one!
Or a time machine… oh wait, I have one!
Or a really big wardrobe that doesn’t take much space on the outside, oh hey, the Tardis’s got one.
Maybe some sort of magical laundry device, oh wait, we got one of those, too.
I guess I could just build something really impressive to steal your girlfriend, oh wait, I got a spaceship, chicks dig spaceships.
Literally, a Tardis is the only thing you’ll ever need, because the thing comes standard with so much technology and everything else will be invented eventually and somewhere, you just need to go there and get it. And avoid ill-tempered bastards in silly hats and robes while you’re at it.