A good wikipedia article on Balkan grammar:

[URL="https://en.wikipedia.org/wiki/Balkan_sprachbund"]https://en.wikipedia.org/wiki/Balkan_sprachbund[/URL]

It seems that Macedonian is the most innovative language out of the Slavic languages, Bulgarian is a close second. The grammatical distance of these two languages to other Slavic languages is outstanding, with the exception of the Torlakian transition dialect.