This sounds like a good project. I haven't read through all of the replies so I may repeat some things.
I don't think a roof mount makes sense. I would guess that most people want to keep the sleek appearance of the exterior of their car. How about a super small camera mounted at the top of the windshield or in front of the mirror? Maybe even in the front grill if the lens has a super wide FOV?
I really like the idea of wifi in a car DVR. Set it to sync to the cloud. If someone hits the car while parked and connected to wifi (i.e. in your driveway) you would have video of the incident even if they stole the cameras and located/stole the DVR. Of course a 256MB micro SD capacity would be a must. Smart alerts to the cell would be great...why let a thief know you're coming for him? The 4G idea is great but I wouldn't want an extra monthly bill. Maybe it could be activated on-demand, sending current and recent video clips or snapshots to the cloud?
Multiple (2-3) 4k cameras with night vision (color?) would be ideal.
Eventually this will hopefully be integrated into new cars. Make a good product, patent it and then maybe you could license it out. I'd pay a few hundred or so for this.