Math.clamp is a big one (it’s a TC39 proposal). I’d also love to have the stats functions that Python has (geometric mean, median, etc.).
On the more ambitious end: CSV reading/writing and IPv4/IPv6 manipulation.
Deno's standard libary has nice CSV parsing/serializing, and you can use it in any environment.
Docs: https://docs.deno.com/examples/parsing_serializing_csv/
https://developer.mozilla.org/en-US/docs/Web/API
The above seems fairly expansive, even if we remove all the experimental ones in the list.