The Hunnic Empire relied pretty heavily on Germanic auxiliaries for administration and consolidation of power in the areas close to the Roman frontier. In my view, this was ultimately their undoing because these groups were inevitably going to see that the Hunnic system was not going to get them what they wanted, which was arable farmland reasonably secure from attack, or offices in the Roman state. The Ostrogoths went the latter route, and other groups the former, but the point is that the Huns regardless of their prowess were overstretched. The Hunnic Empire was far more sustainable in the Eurasian steppe and could have persisted there I think for a long time. Classical civilizations still had the wealth to keep them happy in a raiding/tribute providing role, the topography was ideal for their way of life, and the lines of trade and communication were not as bad as one may think.
The biggest problem that faces all nomadic empires is figuring out succession and the transition from being a conquest society to a post-conquest society. The Huns did neither.
I would not describe the Germanic affiliates as auxiliaries. From what I understand of Gothic myth surrounding the Huns, it would seem that the Gepids and others affiliated them, likely unnamed and unknown Germanic and Scythic peoples, were not just subordinates of the Huns, but partners in empire. The Huns and their affiliates expanded with more opportunistic or subjugated peoples indeed, but these folk were not only united by Hunnic power, but by a greater impetus.
That impetus has been discussed by some authors not as something arising in the Hunnic camp alone, but in a general trend in Northern and Eastern Europe (of which the Huns were a part of, they were not a people arose from Mars as is sometimes felt in the public consciousness). That being that across the Roman borders, the Germanic and similar folk were of differing views and relation tot he Roman empire, some of whom wished to curry favor with Rome and viewed the Empire as the 'Great Kingdom.' They sought to enter into Rome and rule over it as allies with the Great King and institute fair trade and relations as well as serve in its armies. Others wanted simply to loot, plunder and remain above these affairs. However on the other end, a definite side wished to subjugate the Roman empire into a tributary realm and reformulate the hegemonic status in Europe from Rome to another realm across the Danube-Rhine. In this sense, the Hunnic empire formed the critical and formal declaration of the latter, acting as the primary anti-Roman power in Europe and advocating in favor of its vassals and itself as a realm in opposition to perceived Roman hegemony.
In that endeavor, the Gepids were definitely co-rulers. They sought to acquire land in the Roman empire and presumably, to acquire loot and pillage that would be provided for by the wider Hunnic tribute system that would have tentacles across Europe. Other groups, such as the Thuringians and Saxons also benefit, in that they operate as beneficiaries of the Hunnic elites and recipients of the bounties of the transition of Rome from a dominant power into one that is ultimately a subservient tributary to a wider network of Hunno-Germanic coalitions across Northern, Central and Eastern Europe.
The reason the Huns however filled the role that allowed them lordship over the Gepids as the 'head' of the coalition, was that the Huns possessed the greatest power and the most centralized form of governance. During the reign of Hunnic king Balamer the Huns may have managed to integrate the Gepids into their coalition and afterward defeated Ermanaric of the Greuthungian Goths, the prior hegemony in the Pontic Steppe. After defeating the Greuthungi, this signal is what permitted the Huns to ascend to the status of the hegemony in Northern Europe and begin its process of a wider coalition creation that would lead to the reign of Attila and his great successes. All built on the successes of good coalition building with the Gepids and Goths under Balamer, Huldin, Charadon, Oktar, Ruga and then Attila. Once this power was gone, the Gepids had no reason to follow the Huns, nor did the Heruli and others. It would seem that only the Ostrogoths and the Thuringians remained relatively loyal to the Hunnic hegemony up to Nedao and then after, the Huns were unable to maintain their nominal coalition with the Ostrogoths. Afterward, it seems that what remnants of the Hunnic nobility that remained, migrated to and assimilated into the Thuringian kingdom which was reaching its height at the time and was in accordance with the Hunnic hegemony until the very end.
To maintain the Hunnic hegemony, we must remove the succession crisis early. Then, we need the Huns to engage in bloodletting. That is, the Huns must fight someone and win, ideally that is an enemy people, such as the Eastern Empire alongside the Heruli, Gepids and so forth. If they can win this time and restore some legitimacy, they may can maintain their hegemony, ruling as overlords over the region from their base in the Pannonian grasslands.
Their empire would be one of them seated in Pannonia as the ideal masters of the varied coalition under their command. The Amal Goths ruling Dalmatia, the Thuringians ruling the lands to the northeast, namely southern Saxony and Bohemia, the Heruli ruling Austria proper, the Suevii ruling in Carinthia, the Scrii ruling in the southern sectors of Poland, the Gepids ruling the length of the Danube south of Panonia as the great vassals of the Huns and the Lombards and Rugians ruling as affiliates with the Thuringia north of their central parts. Such a Hunnic empire will be very strong in my opinion if it continues to attack enemies on all sides constantly and maintains a constant stream of redistributed loot.
At best, such an empire could become very similar to the Arsacid Confederacy of Iran. With the different Germanic coalition members acting as Great House like powers and the empire otherwise focused on looting the West, South and East and maintaining legitimacy in the form of loot, pillage and battle prowess.