I'm not sure that the Arctic needs "rewilding", it is still a wild environment. What's definitely needed is protecting it.

Mammoths were adapted to the ice age while we are in a climate warming crisis... apparently they lived in steppes called "mammoth steps" that are very rare today [1]. So realistically this would just be a tourist attraction a la Jurassic Park.

[1] https://en.wikipedia.org/wiki/Mammoth_steppe