For example: if the Magyar tribes were assimilated by turkic tribes before arriving in the carpatian basin? Or if Bulgaria either had kept the old turkic Bulgarian language or developed a dialect during the dominions of the Ottomans, but still kept a separated national identity? This and other possibilities.
There's an immediate problem with Bulgarians, and essentially complex and super-complex chiefoms with a Turkic dominant componant in Balkans : they didn't as much replaced or integrated Slavic chiefdoms than they included them into their structure. A bit like Avars before them, this had the distinct result of having Slavic people adopting originally non-slavic indentities (Crovars, Croats, etc.) and in this case Bulgars. Christianization of Bulgaria with use of Old Slavonic was also pretty much, IMO, dooming Danubian Bulgar into irrelevance.
I think you'll need to either prevent some Turkic peoples to enter Balkans, or to have Turkic people with a distinct enough identity and structural basis for this entering in.
In the first case, a good choice would be Kumans/Kipchaks in the Pontic Steppe up to Danube : while they did included Vlach or Slavic ensemble, it seems it was less crushingly so than what happened in Balkans and the odds having without Mongols coming in, at least a distinct Kipchak-issued minority in the region comparable to Crimean Tatars, possibly more, are there.
Conversly, more Turkic peoples integrated within Mongol super-chiefdoms could end with more minorities as Nogai Turks (or these being more present).
On the other hand, Turkic settlement or acculturation in Balkans is fairly possible with Ottomans :
@Koprulu Mustafa Pasha already mentioned prime suspects with Gagauz, Crimean Tatars, Dobruja and Thrace : you could see other places but these are the "best" choices to reinforce and make like viable distinct ethnies (apart from Thrace, which I think is too close from the cores of Ottoman Anatolia to be really considered different IMO).