I read somewhere once (sorry to be so vague, but it was quite some time ago) that in excavations in one of the Danube Bulgars' capitals, the remains of a (supposedly) religious structure had been identified, and that some specialists saw in it some similarities with Zoroastrian fire temples. It struck me as noteworthy then, and now that you mention those Scytho-Sarmatian theories, it can possibly be related to them.
AFAIK there were eternal flames in the temples and some Byzantine chronists stated that the proto-Bulgars worshipped the celestial bodies. It's a bit of a jump though to make parallels to Zoroastrianism or older Iranic religions on the basis of that evidence alone. There are a some (in the single digits) references to Tangra, but all of those are from the time of Krum's dynasty, so it is again unknown if it can be projected earlier. I guess the possibility of fire worshipping Turkic tribes and Tengrist Sarmatians is also on the table. Oh, I also forgot to mention in my earlier post that there are autochthonous Thracian origin theories as well, but those are usually beyond the rainbow and not really taken seriously by academia.
It could very well be so, but it's quite dubious if it will be possible to discern between Germanic residual populations in the Balkans. The Goths were not the only Germanic group in there, at least there were also the Gepidae (and maybe the Herulii too?). Add to that also the remains of the Carpii (of possible Thraco-Dacian stock), Alanic Sarmatians...
It is largely assumed that the Germanic population was marginal by the end of the VII century, with the majority of it moving into Italy with the Gothic conquest. There seems to be a concentration of Germanic toponyms in the general area where the Severs were supposed to be, but those may have come at a later date (i.e. German miner migrants in the high medieval era). The general consensus so far seems to be that the Severs were yet another Slavic tribe, though the evidence for that is more or less the name of a single Sever notable (Slavun) from the late pagan period, during which the population of Bulgaria was culturally homogeneous (as far as a medieval population can be considered homogeneous I suppose).
If you believe the Bulgars were Iranic, then how do you explain the Volga Bulgars, who were clearly Turkic? Since the Volga Bulgars and Danubian Bulgars are supposed to have originally been one people before they split off and went in different directions. The modern (Turkic)
Chuvash and
Volga Tatars are almost certainly (partly) descended from the Volga Bulgars. I'm no expert in linguistics, but Wikipedia says that there are surviving samples of the Bulgar language written in the
Old Turkic alphabet, etc.
Unfortunately, the story of the Volga Bulgars is also not clearly cut. Firstly, the Voga Bulgars are allegedly descended from the Kutrigurs, while the Dunabic Bulgars are supposed to be Unogondur. As I mentioned, it is unclear what the relation between the different tribes was. Secondly, Volga Bulgars did not exist in a vacuum. A cultural fusion akin to the one in Dunabic Bulgaria, but with a different product, is well within the real of the possible. Just like Dunabic Bulgaria had large Slavic population and neighbours and developed a Slavic speaking culture, so did Volga Bulgaria had at least some Turkic speaking population and neighbours. Volga Bulgaria may have also been at least a nominal Khazar vassal for a time. That's another possibility for introduction of a Turkic language, at least as a state language.
Claims of Bulgar origins of the Chuvash and the Volga Tatars should also be taken with a grain of salt. Volga Bulgaria was the established Muslim state in the region. It is unavoidable that, when the modern national identity of those peoples was developed by Chuvash and Volga Tatar intelligentsia parallels to Volga Bulgaria would be made. A bit like the South Slavic Illyrian movement (I guess I need to mention the similarity between
Veda Slovena and
Cäğfär Taríxı here). There is very little direct evidence to confirm that Volga Bulgarians survived the Mongol invasion and re-emerged as Chuvash and/or Volga Tatars. As far as I'm aware no Bolgar books survived to modernity, so that doesn't give any indication of the vernacular or state language. I'm also not aware of any large body of Runic inscriptions in Volga Bulgaria, but that can also be misleading. Most of the pre-Glagolic and pre-Cyrillic stone carved epigraphs, if not all, in Dunabic Bulgaria are in medieval Greek. AFAIK there are some Runic inscriptions in Dobruja as well, but they have not been deciphered so far.
In a way Volga Bulgars are even a bigger mystery than the Dunabic Bulgars. The second at least had Byzantine chronists writing about them.
The Iranic origin theory, bases itself on the fact the the proto-Bulgars (both on the Volga and on the Danube) were most probably semi-nomadic by the time they settled at their end destination. They were building keeps and fortresses, as well as growing some crops. That lifestyle is closer to the Sarmatian tribes on the Pontic steppe in that period of time than to the Turkic tribes, which were mostly still fully nomadic at that point. Though again that's not really absolute proof either way, Turkic tribes could have picked up those habits as well as the Sarmatians did.
IMHO, as a casual reader and not part of academia, the biggest problem with the Turkic origin theory in Bulgaria ATM is that it was the established and accepted theory for about a century, so its proponents prefer to go the easy way and speak from the position of authority. That becomes problematic since the theory, as is, was formulated during the early XX century, when most of the archaeological finds we have now were simply not yet discovered. So a revision that fits the (not really that) new evidence with the theory seems to be in order.
. There's Finno-Ugric theory of the proto-Bulgars as well. The problem is we know close to nothing about the language and religion of the proto-Bulgars, so most of the theories are based on guesswork. Lately, Scytho-Sarmatian theories seem to be becoming the dominant in Bulgarian historiography, displacing the Turkic origin theory, but again the evidence is slim either way. For that matter, we cannot really say if Kutrigurs, Utigurs and Onogurs were culturally homogeneous people or an ad-hoc confederation.
