It is not too broad, but site need some spetification at the description.
There is overlaps with Russian SE, Ukranian SE, Language Learning SE, and Linguistics SE.
There is only 16 (extincted languages exluded) of Slavic languages (Wikipedia):
East: Belarusian, Russian, Ukrainian, Rusyn
West: Czech, Slovak, Polish, Silesian, Kashubian, Upper Sorbian, Lower Sorbian
South: Bulgarian, Macedonian, Church Slavonic, Serbo-Croatian, Slovenian
Separate-SE-lang (Russian, Ukrainian) and no-mainlang-country (Rusyn, Silesian, Kashubian, Church Slavonic, Upper Sorbian, Lower Sorbian) can be move to “more...” section, so there is 8 left:
West: Czech, Slovak, Polish
South: Bulgarian, Macedonian, Serbo-Croatian, Slovenian
̶(̶b̶e̶l̶a̶r̶u̶s̶i̶a̶n̶ ̶i̶s̶ ̶a̶ ̶l̶i̶t̶t̶l̶e̶ ̶b̶i̶t̶ ̶a̶l̶o̶n̶e̶ ̶i̶n̶ ̶t̶h̶i̶s̶ ̶c̶o̶m̶p̶a̶n̶y̶)̶
So, this is SE for:
- question about some group of Slavic languages.
- question about these Slavic languages: Polish, Serbo-Croatian, Czech, Bulgarian, Slovak, Belarusian, Macedonian, Slovenian and more.
- Russian-specific question or Ukrainian-specific question are not belong there (except they compare with other Slavic languages).
Language is arranged by count of native speakers.
languages are too far away from each other to be lumped sensibly,
Polish will dominate over small ones. But it is only way to create SE for these small languages. Also don't forget about question about some group of slav. languages.