just be glad for EU
there are so many factors that make a 'slavic superstate' a thing of mere fiction that it's no wonder it hasn't quite happened yet (and let's be honest ussr was a shitshow, so i won't count it as an example). western, southern and eastern slavs have parted so much over the years that we're essentially different cultures now. sure, we share some holidays, some similarities in our languages, and occasionally mindsets, but for example czech values are nowadays completely different to those of bulgarian.
the economic difference between individual countries would be a huge issue as well, where it'd take a shit ton of convincing for the more developed countries to financially aid those that are a bit lacking behind; as not even the 'prime slavs' could be considered the superpowers we'd love 'em to be.
as some people have mentioned, we have the visegrad four, we have eu; both of which have been helping out tremendously, even if you don't like to admit it. slavic superstate really seems just like a wet fantasy of a typical patriot blindly proud of their heritage in my eyes tbh.