Yes, by this time, the two sides of the Kapela Mountain Range was clearly on a path on cultural unification already, however this happened after/during the migration wave and the unification of the two Banates. It's quite a bit late for this POD.
Well, yes, that was OTL.
Well, I hope you didn't mind that I wrote all that then, since I felt as though some extra regional context could've been used for the discussion.
Basically my goal is to preserve the linguistic boundaries of the region prior the the changes in the XVIth century. Dementor posted a quite good map about it (even though the Magyar population is not showed properly, nvm), but I here's my map, which partially covers this area.
Let's say the Southern fortification system of Hungary doesn't collapse and the Turks remain bogged down there. The borders remain relatively the same until the very end of the XVIIth century, when Bosnia( without Hercegovina), whole Macva and Oltenia is lost by the Turks. Then the borders remain the same until around 1800. After that idk.
Now, I'm not exactly sure how you can preserve linguistic boundaries completely during this time period, though that may be because I'm not a linguistic historian, only knowing some facts here or there.
So, we can assume the loss at Mohács, among others, is prevented. So, we have a time frame between the early 16th century and the very end of the 17th century, which I would imagine depicts a very different period of war between Hungary and the Ottomans, since I very much doubt the Ottomans would stop trying to take Hungary.
The main issue for me in continuing this discussion is, well, my unfamiliarity with the history of the region between the 15th and 18th centuries, roughly the majority period of the Ottoman wars against Europe. But I do know that there are severe implications for the continued survival of Louis II, especially when it comes to the Habsburgs. And while the historical implications of this entire scenario aren't all too relevant to this discussion, I generally like to have a base from which I could stand on when it comes to details such as this.
Ah yes, that map was which inspired me to launch this thread in the first place. However, the Bosnians were always a bit different, werent they? I speak primarily from a religious point of view, which even though their language with the Serbs is almost the same could still lead to a different identity. If they maintain their own church, that's their reason for separation, if they become muslim as OTL, than that's the reason, and if they somehow become catholic, then they basically become the OTL Croats. But if they become orthodox, then that's a whole different story.
Sounds about right(, read my suggestions though, I'm eager to hear your opinion). Maybe my other question, that how close could the Kaykavian and Slovenian languages become?
Religiously, speaking about their history with Christianity, the region of Bosnia, and Herzegovina (which by this point had received its regional name), had always been in some ways behind the times when it came to the latest dogma. The Bosnian Church was considered heretical, but it wasn't Bogomilist or anything as they were accused - they were heretical purely because they hadn't kept up with the developments the other churches were doing and were effectively out of touch (
more info here). The Church itself was heavily connected with the aristocracy and court, and thus when Bosnia fell, the Church itself began to dissipate and merge with the other Christian groups in the region, with some slowly moving towards Islam due to it being more beneficial under the Ottomans, though for most of their occupation they seemingly ended up switching to a syncretic sort of Abrahamic religion, merging Islam and Christianity, leading to situations where one brother followed Muhammad's teachings while the other believed in Christ.
Personally, I feel as though there will be some sort of migration happening for the Serbs, especially if the same set of circumstances occurred this timeline. I'm not as certain about the Croats, but if we have continued conflict between the Hungarians and the Ottomans, there may be a much more minor migration. Still, this does leave us dialectically different for this Hungarian pushback against the Ottomans in the late 17th century.
While I'm not certain about the precise religious situation in Bosnia at that point, the Bosnian Church as an entity is long gone, and there may be a push towards getting those in the region to follow Catholicism, especially by the Pope. This doesn't guarantee that they become exactly like the Croats though. The region has had a very fascinating history up to this point, and any push towards making them more Croatian may result in them adopting a uniquely developed Bosnian identity. This has implications that I'll get into.
With Kajkavian, Slovenian, and we could also add Čakavian, honestly, the only reason they didn't effectively become the same language is because of the separation between Austria and Hungary, at first literal before becoming administerial with Cisleithania and Transleithania. With this POD, assuming the Habsburgs don't eventually take Hungary, we may see either no difference to OTL or slight more divergence between Kajkavian/Čakavian and Slovenian.
So, concentrating directly on dialects only, Serbian is unchanged in its course of Štokavian, though the exact dialect they may use could end up different from OTL, since we can assume Vuk Karadžić to have been butterflied. The way they speak could also end up differently, since we don't know if we'll see Ekavian manifest itself as the dominant way of speaking like OTL or if Ijekavian will win out. Croatian, by way of butterflies, could see a lot less Štokavian TTL, and this may push it towards a Kajkavian-Čakavian mix, though whether they'll speak in Ekavian, Ijekavian or Ikavian I'm not entirely certain. Bosnian, on the other hand, would probably also be Štokavian, though of a different kind to Serbian, and it may pick the way of speaking that isn't used by Croatian or Serbian (so if Serbian is Ijekavian and Croatian is Ikavian, Bosnian could end up Ekavian, etc, etc). If we also take into consideration that Slavonia would lean more towards Bosnian than Serbian when it comes to Štokavian, with the separation of the two banates, we may end up seeing Slavonian and Bosnian coincide with one another. So, the internal Serbo-Croatian languages would be Serbian, Bosnian (including Slavonian) and the more distant Croatian (or we have a Serbo-Bosnian language and a Sloveno-Croatian language, all depends on how linguistically things turn out by the time national identities develop and if we'll see things akin to the Vienna Literary Agreement TTL).
When it comes to the Torlaks, it is very likely that they'd be seen more as Bulgarian than Serbian TTL, but it is also equally as likely that we may see the manifestation of a unique Slavic identity, perhaps in tandem with the Slavic speakers of the Macedonian region. Though this all revolves around how the national identity of the people in the area develops, since at this time in the Ottoman Empire, Slavic national identity wasn't really a thing yet, revolving more around one's religious identity.