What can be used as a connector for the I2C sensors? It should be customer friendly. So something like RJ11 or USB. What can be used for that?
Almost anything you like. Whatever is easy to get hold of and use. You only need 4 wires, and they are all low voltage, so nothing too fancy is needed.
Is it possible to send data wirelessly from a I2C sensor?
No. But you can connect the I2C sensor to an Arduino then use that with a wireless interface of some form to send the data wirelessly.
One thing for you to know though:
I2C is not suitable for long distance communications. A few inches to a foot or so is about the limit. Anything longer and you will start running into communication problems. For longer distance communications you need to use a more reliable protocol.