Southern Slavs mostly identify with religion, so wouldn't it make sense to have Bogomilist Slavs as Bosnians?
Catholic=Croat
Orthodox=Serb
Bogomilist=Bosnian
I don't think Paradox wants to add a small Christian heresy into the game though, I wonder how could we represent the Bosnian Church without adding new features etc.
Maybe, but like you say Paradox wouldn't want to. If they were going to do this, Bohemia would have Hussite provinces, especially as Hus was a much more important and well-documented part of pre-reformation Christianity. Not to mention, we really don't know how prolific the Bosnian Church was in the medieval Kingdom of Bosnia. Only one Bosnian king seems associated with the Church, and the rest are either Catholics or Orthodox (the last king was definitely Catholic). So if they had provinces be Bogomilist/Church of Bosnia, they would be converted away unless the Ottoman's took Bosnia fast enough.
If we get a Balkan update (add another Slovenian province in Austria, update Croatia/Bosnia/Serbia/Albania/Greece), I could see something perhaps being done but it's such a small country.
Anyways, yes, South Slavs tend to identify as religion, but I don't know how much of that is historical. The people in Bosnia would have referred to themselves as Bosnian's but that's more that a geographical/national identifier than cultural. Culturally, the people in the Kingdom of Bosnia were Serbian.