If the US dollar crashes, alot of major companies will be affected. They are almost all based in the US, meaning that their funds will mostly be kept in USD, but not necessarily on US soil. Their holdings will plummet in value.
The USD plummets and you'll see a scramble by a ton of countries to try and bully it out of the world reserve currency position, which might be possible with the right circumstances.
USD debt will be worth nothing, massively affecting countries with large bags of US debt, namely China. This means a ton of assets internationally are seriously affected.
This also means that the US would likely face a collapse, considering they would now suddenly have the currency of an export country in an import country. Which is a very bad mix.