I think the only downfall to a Hetfield signature amp is that Hetfield himself would be the person least interested in it. Or at least the least interested in the version we all want.
The JP-2C came about in no small part because John insisted on them doing a remake, because he loved that particular amp so much and wanted more. And wanted newer equivalents to use on tour without risking his classic ones. John also still mostly favors the sound he's used his whole career.
James, on the other hand, went down totally different tonal rabbit holes, different amps and sounds, and his modern tone is nothing like it was a decade ago, let alone 30 years ago when he was making THE CLASSIC SOUND that the fans would want from a signature amp.
So John really wanted an amp, that looked and sounded just like the amp fans of him would want, and pushed for it. James seems less interested, and if he did want an amp he would probably want something that isn't at all what the fans want, and instead it becomes Mesa bugging him to make one on behalf of the fans that he might not even want to use much.
That's my Debbie-Downer two cents worth of rambling on why my hopes aren't high