The alternative is that the Bulgars appeal to the Avars for help. Now if that happens they would have to enter a dependency relationship, but it would essentially act as a buffer state. The problem is that the moment the Avars are distracted elsewhere Bulgaria is on its own, and they could achieve OTL levels of luck, or they could get conquered, or somewhere in between.
Well, I didn't reply to that post earlier and I can't edit my previous post any more to maintain that reply, so I am creating new post:
a) if Bulgars appeal to Avars for help, IMHO Avars would absorb Bulgars entirely in reverse what was done OTL, considering that Avars had already sizable population speaking that dialect of Proto-Slavic which gave birth to OTL Bulgarian (even the name of half of Hungarian capital "Pest" means "cauldron" in Old Slavic Bulgarian) so probably Bulgaria will be bigger ITTL if that option happens and called "Avaria" instead of "Bulgaria"
b) if they don't, Avars become Byzantine problem once again and they proved themselves to be troublesome neighbours even before they had any influence south of Danube, so IMHO Byzantine-wank going up until full reconquest of Middle East requires Byzzies mounting anti-Avar coalition and if not eradicating them entirely like it was done IOTL than at least reducing them to rump state unable to threaten anyone
Also wouldn’t an avar survival mean the Magyars don’t arrive in Pannonia? If so, I think another Slavic kingdom like Samo’s kingdom would be established, what effect would that have?
Samo's kingdom wasn't located in Pannonia (Samo merely invaded it already after gaining power among Slavs and it's unclear whether he subjugated tribes living there or not), it's basis was so-called Bohemia proper (western part of Czech republic) or Moravia, tribe of Moravians was Avar sort-of-vassal before Charlemagne invaded and I don't see them liberating themselves without outside help, Bohemia proper was ruled by prince Lech, who OTL got murked by Frankish forces and was overall three of most relevant Slavic princes resisting Charlemagne - besides him there were also Drogovit, duke of Veleti and Miliduch, duke of Sorbs, Thrasco/Drozhko and his precedessor Witzan, duke of Abodriti were allied with Charlemagne, and probably ITTL Obodriti will get destroyed by Danes, Saxons and Veleti, but Veleti would be threatened by Danes and Saxons, so maybe they would seek alliance with Bohemians, as they did later when after Bohemian unification led by Premyslids, which might lead to Bohemians, Sorbs and Veleti forming larger tribal union (it could be comparable to Samo's kingdom, as it was also anti-Avar tribal union), if Bohemians feel threatened by Avars enough and Veleti and Sorbs by Saxons, maybe even encompassing some tribes from OTL Poland - if the certain theory is true - the name "Lech" is also attributed to legendary founder of Polish state, and chronicler Einhard claims that Charlemagne subjugated all peoples living up to Vistula - "
deinde omnes barbaras ac feras nationes, quae inter Rhenum ac Visulam fluvios oceanumque ac Danubium positae, lingua quidem poene similes, moribus vero atque habitu valde dissimiles, Germaniam incolunt, ita perdomuit, ut eas tributarias efficeret; inter quas fere praecipuae sunt Welatabi, Sorabi, Abodriti, Boemani - cum his namque bello conflixit -; ceteras, quarum multo maior est numerus, in deditionem suscepit." - in English: "therefore he subjugated all barbarian and ferocious tribes dwelling in Germania, which live from Rhine to Vistula and from ocean [Baltic Sea] to Danube, all of them very similar in language, but sometimes dissimilar in customs and made them tributaries, among those most important were - Veleti, Sorbs, Obodriti, Bohemians, with whom he had made war and whose are very numerous", so some historians, like Henryk Łowiański (Początki Polski/ "The Beginning of Poland" t. 4, Warsaw 1970, s. 399) claimed that Bohemian prince Lech extracted tribute and men from part of tribes living in what would later become Poland and thus was immortalized in legend as founder of Polish state, but I don't know if even if that tribal union would be established, even in most optimistic scenario for them - encompassing Veleti+Sorbi (most of lands from Elbe to Oder) + Bohemia proper + parts of OTL Poland, going directly against Avars might be seen as too dangerous, especially as there are still Danish, Saxon and Bavarian threats, and at the time of probable formation of that union (820s-830s, approximately time when IOTL Great Moravia arose) no Magyars are in picture and thus it has no bearing on their arrival, and Avars might get pretty Slavic themselves by that time, IMHO only instance in which aforementioned tribal union would feel strong enough to invade Avars would be if there was coalition against them with Byzzies and Lombards at helm, otherwise they would prefer to subjugate rest of what became Poland IOTL (shitty lands, compared to those on south, but easier to conquer + subjugating
https://en.wikipedia.org/wiki/Truso and
https://en.wikipedia.org/wiki/Wolin could give them plenty of money from trade) and even if that panned out they would probably prefer to harass Eastern Slavs and set themselves up as rivals to Rurikids than risk confrontation with entire Avar might - approximately that could happen in 880s and that's when Magyars enter the fray, tho it's not even set in stone they would invade Pannonia - they could do so still, especially if Byzzies set up anti-Avar coalition, but they might as well stay as Khazar vassals if Rurikid-"Western Slavic tribal union" rivalry would benefit Khazars, they might invade some other place (like Armenia in excellent Carantania TL by
@SealTheRealDeal ), they might invade Pannonia but fail and get absorbed like remnants of Alans IOTL.