I think I've found all of the remaining rewards for secret challenges / easter eggs in Warzone although some of these could be released in the future for different things and not tied to secrets around Warzone.
- Shattered Vision weapon decal
- Blood Vials charm
- Golden Spork charm
- Dragon Wings charm
- Rocket charm
- Shards charm
Apparently there are some strange loot crates on all maps that have the red mist from the Black Ops 7 teaser secret challenge. They've made a comeback it seems but they could either be bugged or just insanely rare to find.
There's also the possibility of another easter egg on Rebirth Island that is even more hidden than the Gulag one. Still trying to piece together some clues.
Interesting nonetheless!