Part 1 was showing everyone what the car looked like and the powertrain, this was a very car-centric reveal. Part 2 is going to be about the things that weren't discussed and that's namely the Autonomous capability and all the other electronic related stuff. Think part 2 as more of an iPhone event.
Elon basically gave away one of the secrets to part 2 which is there will be a futuristic steering system, most likely a steering wheel with a flexible OLED touch display. That's why there was nothing to been see on the dash. HUD is old, not interactive and not suitable for the environment of a car, let alone with that much glass. And lets be very clear, a car that is for the masses is not taking out the steering wheel on the first try. Full Autonomous driving features will ramp up slowly for both safety and legal issues, so in the meantime don't expect any car coming out in the next 1-2 years to have no steering wheel. The robotic memorized line from all the Tesla employees during the event about the 15-inch replacing the dash screen "Giving more access to the passenger.." was an obvious smoke screen to avoid talking about the real plans.
You can bet the Tesla OS is going through a major overhaul with a bunch of new integrated features. Autonomous driving alone requires a lot more features and to be a lot more integrated. It will most likely introduce the ability to enter destinations and work with the map. The steering wheel touch display will probably be way more integrated then the dash display ever was. It will probably be able to switch into different modes like from navigation to media mode and probably allow some customization's. Depending on the size of the steering wheel touch display there may be either very few physical steering wheel controls to zero. Meaning everything from turning signals to wipers could be controlled from the interface if they choose to go that way. This also means that the display will have some sort of image stabilization relative to the movement of the steering wheel. Since this is a vehicle for the masses we can expect there's going to be an integration of new services through partnerships that will also show up on the updated OS. This will allow Tesla to make more money on the Model 3 through value added services.
There might be some other stuff but I expect this to be basically what we will be looking at when Part 2 comes around.
**I have no insider info and this is speculatory**