Add baltic languages to the mix as well! Lithuanian is like a slavic language with all the inflection drama but with additional word types that are currently mostly gone from slavic languages.
The language seems to have more archaic features and forms than, say, closely related slavic languages, and its vocabulary has more similarities to old sanscrit than one's average european language.
For curios language learners this means that the grammar is harder than even (already hard enough) slavic grammars.