This has probably come up before, but I'm in a speculative mood.
If, theoretically, there was to be a new Heavy Jack Kit for the factions, what Chassis would they be based on? It sounds fairly obvious for most factions:
Khador: Devy Chassis W/ Devastator, Spriggan, ???
Cryx: Crabjack Chassis W/ Leviathan, Harrower, ???
Cygnar: Centy Chassis W/ Centurion, Hammersmith, ???
Menoth: "Homebrew" Chassis W/ Castigator, Reckoner, ???
But, then, what about a Merc one?
The question is whether Rocinante is an Upgrade kit for a brand new Merc Plastic Kit or a brand new $50 sculpt. If, theoretically, it WAS an upgrade kit, then that kit would have 3 jacks it could make. One, obviously, is the Nomad, and the third would be something entirely new...
So what existing Jack would make it into that second slot?
You have the Mangler and the Mule, but these have stark differences in chassis to the Nomad, whether by Boiler design or simply bulkier armor. If either of these were to be used, there would be some....not heavy resculpts but more complicated resculpting to more match the Nomad's appearance.