There’s actually a pretty simple criterion for what kind of business/industry should be nationalized: The good or service it provides is something the government believes should be guaranteed to people as a human right, not tied to a profit motive.

People have a human right to healthcare, electricity, and access to the internet, so those should be nationalized. People don’t have a human right to McDonald’s hamburgers, so that doesn’t need to be nationalized. Sometimes there are grey areas, like everyone has a right to shelter, but not necessarily to a luxury condo. So there’s room for some public housing as well as some private development.

Separate from that, competition should be improved by breaking up companies when they get too big. Countries used to do this a lot. Teddy Roosevelt did it all the time. It’s called trust busting. A company like Bell or Loblaws gets told they need to sell off their holdings until their balance sheet is below a certain size. It also goes hand in hand with actually enforcing laws which punish anticompetitive behaviour like price-fixing.