"Displaced" - a human wearing a costume of villain is tricked by nebulous "merchant" into buying one last thing that would complete the costume. Ends up in Equestria as a villain they tried to depict, has misfortunes. A popular trope to explore around here.

So… what can I do to give it a fresh twist? Well, how about using a good character for a change?

A girl comes to costume party dressed as a cartoon character, buys a pendant cheap, ends up in Equestria. The catch? She`s now a human-sized overalls-wearing scatterbrained high-IQ goodooder by the name of Gadget Hackwrench, and she can build anything out of anything.
The catch? The girl herself is… a little less then sunshine&rainbows minded. Not specifically evil, but… what could you really expect from a girl who grew up in the former USSR during wicked nineties?