So, if a taxi comes to pick you up, do you really care if it's made in Germany with their featured design or it's some econobox? I think I wouldn't care much. Yeah, it's nice if it's roomy and slick, but if not, then not a huge problem, this is not my car.
So, the future will be more about software and less about outer shape of the car.
What will not matter in a robo-taxi is performance. I would imagine there would be a spectrum from cheapest (Model 3 backseat) to more luxurious. Imagine first class luxury full reclining, eating tray, large screen for movies, superior sound system or alternatively office workspace (for that 99.9% of the population who are harder working than myself).