A common slavic language is often criticised because it is seen as one language (usually Russian) being imposed on others. I also believe that it would be a pity to lose all the variations of our original common tongue.
How about instead of a shared language we develope a unified alphabeth, based on OCS, in which the same letter would have a different sound in different slavic languages, depending on what these variations developed from?
For example in Slovene we have the word “volk” (pronounced as vowk), in Serbo-Croatian the same word changed to a U, so “vuk”. We could use a same letter to write this, but in Slovenia we would be taught to read it as “ow” and in Croatia as “u”. I think this could aid comprehension greatly and teach us the history of our languages through the languages themselves. It could get fairly complicated, but there are much tougher writing systems out there that people get used to.