Just need to finally abolish inches of mercury for pressure and temperatures in Fahrenheit.
It is interesting that temperatures in TAFs and METARs in the USA are in Celsius. I'm not sure when that was introduced.
It's unlikely that hectopascal would ever be introduced in the US though - that would require replacing every GA altimeter - roughly 150,000 of them!
The same logic applies in Canada, where we have been officially metric since the 70s.