I do like what the Zimov's are doing in trying to bring back the mammoth steppe ecosystem.
I just feel like they're focusing a bit too little on all the animals that aren't giant megafauna which made up the mammoth steppe ecosystem.
Pleistocene rewilding involves more than just bison and resurrected mammoths y'know?
So what animals would you introduce to pleistocene park if you were able? I've picked four bird species as examples just to get the ball rolling.
Cinereous vultures are the second largest vulture species in the world and are also far more aggressive (and cold tolerant) than the andean condor which would make them a great cleanup crew. This problem becomes even more of an issue when you realize that corpses in the cold decompose way slower, and that hypothetical cloned mammoths would need a lot of vultures to get rid of the meat and prevent disease.
This bird already lives near pleistocene park, so introduction should be fairly easy, and it serves as an invaluable apex predator in its ability to hunt smaller mammals, birds, and even medium sized mammals like deer, wolves, and mountain goats.
This bird would act as a proxy for the extinct asian ostrich and/or pachystruthio, which was a bird that lived throughout the mammoth steppe thousands of years ago. Reason why actual ostriches couldn't be used is because they are not nearly cold hardy enough to survive in northern siberia, while rhea are far more cold tolerant.
This giant bird would function well as prey for medium sized carnivores, and as a way to keep the water vegetation levels normal with all the introduced mammals creating tons of fertilizer.
There's a ton more animals you could bring in like striped hyenas, eurasian beavers, eurasian otters, siberian tigers, eurasian boar, wood frogs, wels catfish, etc. Possibilities are pretty endless.