There could be anything underneath all the permafrost, including new biodiversity which we haven't catalogued yet. NPR ran a story[0] where Russian animals such as reindeer infected with Anthrax had thawed out, and caused outbreaks where people had to get preventative treatments against it. As a future threat model, what if there's new viruses or bacterium contained under previously frozen earth for which modern medicine isn't prepared for?
[0]https://www.npr.org/sections/goatsandsoda/2016/08/03/4884009...