From what I’ve read, the 3 Slavic families are mutually intelligible within a family, but not between. Edge cases exist eg in the borderlands between Poland and Belarus/Ukraine.

So this could go a long way towards closer Slavic relations. In the communist days it was common for the northern slavs to vacation in Yugoslavia for example. Maybe a language like this could reignite a sense of unity, but divorced from Russian hegemony as it was in the past. Wishful thinking however.