I normally use related fields only if I need some value from a parent model (M2O) to use it a view. Like when using domain filters, making fields invisible/readonly, itd.. Else just use M2O fields. Even with related fields you need a M2O field to use it in relate. 

For your example I think you can't the way you want. The res_country_state model has the country_id, so you could get the country name from the res_country_state model and not the other way around. 

country_name = fields.Char(relate='country_id.name') 

You do not need to create related field you just use many2one field to access it.