Alright, before you go saying 'that's got to be the dumbnest thing i've ever heard. Why would a gnome have a connection to the elements?'
And you'd be right. It is pretty dumb. But so are goblin shamans. If we can have goblin shamans, I'm sure blizzard can come up with an explanation for mechagnome shamans. Just say it's super advanced tinkering, like gnome priests and how they're medics and whatnot.
Just look at these glorious baby ball busters:
They actually look like mechagnomes!
They just need to get a couple more colours for the prosthetics, like red and blue and green and you'd be able to make a ton of cool tmog.
Honestly, it's the only way Blizzard are going to get people to play them, if they even get added (which they probably will).