Not really a mod, it's official content. Truth is, we don't know, and the devs probably don't either. They work on one car at a time, we just got the Barstow, and next is the ETK, so we can expect another car to come down the line after that's done, which will probably be in the new year.
Got the Burnside coming after the ETK probably. ETK has been officially teased, whereas the burnside has had no official teaser yet.