It'll look like the Volvo XC40 SUV but with a rear facing 7 seat Model S "option". The development work is already done for it. Might as well use it. They took the option off of the Model S to push people towards the Model X, but also so they could then put it on the Model Y to better differentiate it from the Model 3. This then allows people who cannot afford the Model X a 7 seat Tesla option. And because the 2 additional seats are only accessible through the back hatch there is logically no need for the expensive falcon wing doors as it's primary purpose is not needed.