What are the required security properties for salts?
512 bits is overkill for even global uniqueness. 256 bits should be enough, with a good random number generator: even if every person on earth generated one every nanosecond, it would take a million million years to reach the birthday bound.

However, I agree that global uniqueness is a good idea. If anyone anywhere uses the same salt and password hash (or other salt-using algorithm), an attacker could theoretically attack those in less time than otherwise.

If you had a unique application-specific pepper (additional salt that is shared by all users), you could safely use salts that are only locally unique, for example outputs from a counter. This might in some cases be useful for saving space.