The problem with mild hybrid is that it is not performance oriented (plug in hybrid is) but the PHEV variant can’t be shared with the 1500 because the 1500 doesn’t have packaging room for batteries.
The other problem is that you can overcome the limited performance of the 5.7 by instread building the system around the 6.4, but the 6.4 is more expensive and would make it more exclusive
The economics that make it work is if it can be shared with the 1500, Grand Cherokee, and Wrangler
So that is my guess, the mild hybrid would be based around the 6.4, or the rumored 7.x L HD powertrain engine.
Wouldn’t be surprised if Viper is based on STLA medium, PHEV, and built overseas. Latter half of this decade.