I think you misunderstand how nations came to be. Concept of nation as we understand it was created in 18th century. Before that for peasants your country was whatever the whoever ruled the land said it was. Later the identification became centered on national identities as a lot of nations didn't exist on a map. Largest parts of national identity were the common the language, religion and history. Of those three Belarus only had religion in common with Russia. And the language was forced on them after the fact. (No, I don't think they were slaughtered to give it up. Maybe just the elites but that was pretty common. It was more like what happened to Indians and Aborigines. They were put in schools that only allowed Russian and were punished and made fun of for speaking their language. That is oppression too.)

The fact that the people of different parts of Rus spoke the same language hundreds of years ago doesn't mean they should be one nation now. Once all Slavic tribes were one people and spoke one language and now there are many countries. They developed different cultures and different languages. So did Belarus and even more Ukraine. After all when they were conquered by Lithuania Russia in modern sense didn't even exist. After 600 years spent as part of Polish-Lithuanian Commonwealth one could claim they are really Polish and should rejoin Poland.

Why do you think they will give up independence? Do you really think that the common ancient history can overcome recent aversion? By the same logic Austria and Germany should be one country. And how could USA be ever formed - they were just English who one day decided to have their own country - how could they? They should rejoin UK - they still speak the same language (mostly). And the fact that Kurds never got their own country doesn't mean they feel any less strongly about being separate nation.

I don't think such unification can end any better then Yugoslavia did.

The argument that the land historically belongs to any nation is not a good one in Europe. If everyone claimed all the land that ever historically belonged to them we'd just get another war.