The changes in military law that go into effect 1/1/2019 (UCMJ) represent the most compelling reason for the delay in declassification of the FISA warrant applications and the email/text chains.
I have wondered if Trump recalled Mueller to active USMC duty back in 2017 after he signed the EO allowing the POTUS to recall any military officer, no matter how far out from detachment, back to active duty. This would put Mueller’s actions under the UCMJ. It would also create an incentive for Mueller to wrap up his work before 12/31/2018.
That's an interesting thought.