There's nothing for the Supreme Court to litigate until after new rules are proposed, public comments are made, and then a final rule published in the Federal Register.
With the ATF's recent loss on their unilateral bump stock rule, it's clear that the ATF must pursue the full NPRM process.
And the rules that will be proposed will simply codify the ATF's "soft rules" on pistol brace length of pull, and overall weight of the pistol, and what accessories can be included in an 80% frame receiver "kit."
Not exactly Earth-shattering...yet.
Asking for a friend.