Indeed. People are simply assuming left and right. Knowing the game developers, I expect a mish-mash of various concepts, not just "a Beetle" like everybody's saying.
It'd could be alike to the SBR4, where there is no details that point to one exact (or close) vehicle, as it was more a cornucopia of designs mashed together.
Panamera concept, Nissan GT-R.... Unlike all other officially released vehicles (and the WIP announced vehicles), the SBR4 is unlikely to have a actual real life counterpart, and just really a full-custom design from gabester.
It takes very few cues from the GT-R, some cues from the Panamera (mostly the concept), some cues from the R8, some cues from the tC, and other possible vehicles.
If it's inspired from other vehicles, then it's not original, is it? Although I can agree with the inspirations you have listed
Then, can you find a real vehicle that fits most criteria of the SBR4? No, so it's somewhat original because it doesn't have a distinctive counterpart in RL unlike other other vehicles in Beam.
Here is a picture of the unknown vehicle's engine and the Beetle engine. From this angle, they look very similar. Notice how the curve is for the hood/ trunk line. They are both fairly similar. You can also notice some more subtle similarities, such as the shape of the engine bay (I know that it is a bit difficult to see, but the beetle has a fairly similar shape), the dimensions of the engine in relation to the engine bay, and several other similarities. My prediction is that we will get a vehicle that is similar to the Beetle. Maybe a bit newer, but no newer than the 60's. Whatever it may be, I predict it to have a Beetle shape to it, opposed to a sporty coupe (Karmann Ghia).
Hopefully another screenshot of the new car will be released eventually. At least then this debate over what it's based on could finally be put to rest.
I hope the devs will make a drag version with the flat-4, a V8 might not physically fit without firewall modification.