This picture of SRT's future pretty much answers the "halo" questions. The Viper will remain the halo car until at least 2019.
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First, I think it's incredibly awesome that Dodge/SRT would produce such an insanely-powered car. We really are living in another golden age of the automobile.
I agree with the sentiment that the N/A motor may be finished. The next iteration of the Viper would have to have an increase in power, but there are only so many things Dodge could do.
1) An increase in displacement would make for higher rod loads and internal stresses. Switching to titanium alloys for components that move will not help cost out much, so if that's off the table, an increase to nine or ten liters of displacement would have a lower redline.
2) Maintain displacement but increasing cylinder count would decrease the mass each part, but allow for higher revs.
3) Decrease displacement, decrease stroke, and increase RPM, like the V10s in the LF-A, R8 V10, and Huracan.
4) Forced induction. Same size motor, or decrease the cylinder count and/or displacement to make the overall engine smaller. I think the Hellcat is too tall for the Viper's chassis.
Building a brand new engine costs a huge amount of money. The block would have to be some kind of derivative of something existing.
A front-engine, rear drive V10 is pretty much the signature of a Viper. But just as you can now buy a blasphemous four-cylinder turbocharged motor in a Mustang, you may see a V6 Corvette in the future, and the Viper may have to come with something else in it. SRT will find a way!!