Looks like a connector of some sort. I don't think you would plug a cable into it though. Maybe an accessory of some sort will slide into the connection. One would think it is some type of flash connection or accessory, as I don't know why it would be on the flash shoe. A non-flash accessory would render the flash shoe useless for any sort of trigger or strobe. I certainly hope it's nothing that might interfere with third party triggers for studio strobes.
I'd think it's most likely for video features like an XLR audio output, or perhaps an external recorder or display of some kind. Nothing that would interfere with using a strobe, since flashes wouldn't be useful for video.
GPS is apparently (possibly? I forget if it was confirmed) in the camera, so I doubt that would be for this. Wi-fi they pretty much advertised as being full featured in-body, so I can't imagine they'll market a WFT for this.
If there's a solid connector that clicks in, it can probably push some data out of that, so I do wonder what Canon has planned. I doubt Canon would get into making their own external recorders, but you never know. It doesn't seem like other XLR top handles need a solid plug like that for just pushing audio data.
It reminds me of the small plugs that would be on the bottom of cameras for the battery grip attachment.