I think Slavs should be represented by ..Slavs. ...
Its funny tho.. we`re the most numerous "culture group" in Europe, but can hardly go past 2 civs per mod..
It might be kind of fun to have mod that went, say, from Scandinavia in the west to about the Urals in the east & maybe southern border around the Balkans. Hanseatic League vs. Boyars, Huns, Lemon Kinda (

no offense) lookin' for sumpin', etc.
Why not have me construct two or three BIQs at the same time, working civ-by-civ and deciding as I go along which BIQ or BIQs it should go in? That way, I can include a number of civs over the different BIQs....
there'll need to be a smaller folder for each BIQ in order to house the relevant pediaicons.txt and any cities that would be different from the norm (e.g. african culture to replace, for example, asian culture). This seems like the best way of doing it to me.
It sounds like if you do it that way, it will be possible further on down the road to mix & match culture groups from the various biqs with not too much work in the editor; although the 'pedia might take more effort.
I was also thinking that you could do a "Silk Road" version, with the Asians, the Indians, The Persians (etc.), the Slavs, & a group with the Mediterranean civs (Byzantines, Greeks, Romans, etc.).
If you do end up with room for a 5th Indian culture in one of the biqs, I'd nominate one of the dynasties of the
Western Satraps. They'd cover the one part of the subcontinent you don't yet have a civ from - the western coast & Deccan Plateau. The major trade with Rome to northern India was through their territory. The area is more well known for the Marathas & Rajputs who came later - but similar units could be used (desert cavalry & camels, a navy to match the Chola, etc.).