That is an excellent question @mhkhani.
While I thank everyone for sharing information about open data policies, I was really asking about a possible open data law. Brazil already has a national Open Data Policy, institutionalized by a relatively new Decree 8.777/2016. Some Brazilian cities (e.g., Porto Alegre, Curitiba, Rio de Janeiro) also have open data policies of their own. Besides, guidelines on open data policies are relatively easy to find.
A law, on the other hand, offers the opportunity to be more a lot more far reaching. For instance, in countries that have independent states, the Constitution might allow for a federal law to guide the local law, depending on what subject the law is about. That is the case in Brazil, for instance, with the Access to Information Law (Law 12.527/2011). Besides the federal one, there are hundreds of local (state and municipal) Access to Information Laws in Brazil.
In terms of open data licensing, for instance, while a Decree (or a policy established by a Decree) may make open data licensing mandatory or automatic for governmental institutions, a Law, on the other hand, can determine that some of the possible legal restrictions on the use of data (such as a copyright or a database right) does not apply when the rights holder is the public administration. This is a much stronger form of open licensing, one that cannot be easily revoked when the person or party in power at the moment changes.
This is just one example of something that would be good in an open data law. I was hoping for more examples specifically of things that should be stated in Law instead of just a policy or Decree.