Of course, Bulgaria is a Slavic country today, and I've read that the Avars quickly adopted the local Slavic language as a lingua franca for their domain, and I'd guess it'd be likely that there would be a Slavic people going by the name "Avars" today were it not for the Magyars, who invaded the region a few centuries after the Avars. But what's odd is that the Magyars didn't adopt the local language, but instead replaced it with their own. Why was this so? Why did they differ from the other nomadic steppe peoples who had invaded the Balkan area before them?
the Avar Khaganate of Pannonia was established in the 6th c. and lasted till the early 9th century. hungarians conquested KoH/Pannonia in the late 9th century. it was Charlemagne and Krum of Bulgaria who ended the Khaganate of the avars.
we know very little about the avar culture, and what we know are mainly theories. think the proverb, "disappeared, like the avars". avars adopted/utilized the local language and culture when they were new to Pannonia. but so did hungarians.
lingua franca is used to make people with different native-languages living next to eachother understand eachother better. magyar language preserved the evidence of the contact of hungarians and slavs from the conquest era.
would be a Slavic people going by the name "Avars" today were it not for the Magyars
only if the avars would have been more successfull against Charlemagne and the Conquestors, and less successfull against slavic people.
its hard to tell what happened to the avar people after the Conquest of Hungary, and how their language realted to pre-magyar or slavic. some theories say the nomadic steppe people understood eachother well (maybe they used a now-extinct lingua franca, maybe it was the avar language, who knows) on a daily basis. they were so alike, it was hard to tell apart different tribes only from language and appearance.
the difference you are looking for is the success. hungarians were by far the most successfull conquestors of the Carpathian Basin, and magyars were the strongest tribe among the conquestors. any other hungarian/avar/slavic tribe with the same success rate could have easily 'assimilated' all the other nomadic tribes in their neighbourhood.