If the internal steppe politics go against the Khazars' favor, they could end up migrating to the Pannonian Plains. Either the Magyars, some other Onoghur tribe or the Bulgars end up creating the OTL Khazar-sized empire instead.
Actually Khazars (several tribes called the Kabars) did join up with the Magyars in the 9th C. after a rebellion in the Khazar Khanate. So we do wind up with Khazars in Pannonia. Just, also with Magyars. http://en.wikipedia.org/wiki/Kabar
There were so many Turkish tribes which were a part of the older Göktürk confederation who could have been the force that set up shop on the Caspian in lieu of the Khazars.