Since Plague Marines lost the feel no pain, it would make sense for us to get that since it would no longer be "stepping on their toes" as far as shared special rules goes. Hell, it might even be ignore wound on a roll of 4+ just to be extra spicy. Would work for large damage and small arms AND be fluffy. They have that 4+ ignore flesh wound in Kill Team so they understand Rubrics are just dusty bois.
Anything is possible at this point
I've also been pondering the same thing. This is something I would have wanted previously but not if death guard claimed it first. a 4+ would be fantastic with the added caveat that the 4+ ignore wounds cannot be used on weapons with strength 8+(double the toughness). Which is more or less what it used to be like back then. Representing smaller wounds uselessly passing through the arnour with no body to hit. And only grievous wounds, enough to obliterate the armour could take down a rubricae.