I guess that the next coutnry to accept Bitcoin as a legal tender will be a country from Latin America,Asia or Africa.Underdeveloped countries would want to adopt a currency,that is more stable than their national currencies and a financial asset,that is widely adopted worldwide.
What makes you think that Serbia is the next country to adopt Bitcoin as legal tender?I know that many Serbians are active in the crypto world,the Serbian local forum in Bitcointalk is pretty active and lots of crypto businesses were launched by Serbians.
However,I think that Serbia won't accept BTC as a legal tender,because the Serbian national currency is stable enough and Serbia doesn't have the problem,which El Salvador has-lots of people not having access to banking services