The future of the Third Reich heavily depends on who succeeds Hitler. If he has any children, they will no doubt be a crucial factor in Germany's future and would probably be the new Fuhrer somewhere down the line.
A Goering-led Germany would have the brightest future. He wouldn't desire any more war and likely would try his best to reach a permanent detente with the West. So the likelihood of another global war is very slim with Goering at the helm. He probably would limit military spending in exchange for bringing more luxury to the German people, but also himself. He would still certainly loot and profit from the Eastern Kommissariats, but also wouldn't be as draconian towards their Slavic slaves. Nazis like Bormann, Streicher and von Ribbentrop would've definitely been purged in the aftermath of a Goering takeover, while Speer, Goebbels and Hess would've been demoted or sidelined. Party officials like Lammers, Sauckel and Klopfer likely remain to keep the party apparatus functional. As for the SS, either their leadership submits to the new Fuhrer or they'll be purged. Goering won't risk the possibility of the rivalry between the SS and Wehrmacht of boiling over, and he'd recognize both Himmler and Heydrich as potential threats. And as Hitler's designated successor, Goering would have the clout to have both SS leaders terminated. But he would still need to keep the SS around, so he would have a more loyal Nazi like Kaltenbrunner become the new Reichsfuhrer. Goering would probably have someone like Rosenberg be the Deputy Fuhrer, since he wouldn't be a threat but also isn't a lunatic like Hess. Before his inevitable death, Goering would likely designate his daughter Edda to be the new Fuhrer. Of course, the length of his rule depends on his morphine addiction and his obese figure. But Goering's rule would be the least bad outcome for the Third Reich.
I have nothing to say about Hess. He'd just be a loony figurehead for Bormann or the SS. Moving on.
Speaking of Bormann, his tenure would be a disaster too. He may have connections, but he doesn't have any charisma or clout to be the new Fuhrer. And he has made WAY too many enemies for any of the bigwig Nazis to ally with him. Best case scenario is that he has Hess be the new Fuhrer, while Bormann would either be the new Deputy or become the president of the new Nazi senate. But we've all seen how that story ends. And Bormann doesn't have the power to purge anybody, since that would mean having to collaborate with his rivals. The German people would have little reason to support Bormann, so they likely won't care too much if he gets overthrown. Case in point, Bormann operates in the shadows for a reason.
A Speer-led Germany could be a semi-bright outcome, but it wouldn't be nearly as seem-less as it would be for Goering. He too would try to make peace with the West, so war is a slim possibility. Like Hitler, Speer is an idealist and would probably spend too much money building ridiculous monuments. Not only does Speer have bad blood with the Gauleiters and party apparatus, but he has fierce rivals in Goering and Bormann, and neither of them would help Hitler's architect. Goebbels does have a working relationship with Speer, but their viewpoints are too fundamentally different to work long-term. Speer's best bet is to ally with Himmler by making him the new Deputy Fuhrer and increase the power of the SS, in exchange for greater stability and security. Speer had a disturbing history of working alongside the SS, even with the most insidious crimes committed by the Reich. With the duo of Speer and Himmler, they would successfully be able to purge their rivals and deter Heydrich from taking over, while also placating the Wehrmacht. So Speer's rule would benefit the Third Reich, but it would require him to sell whatever is left of his soul to the spectacled devil.
Speaking of Himmler, his ascension would be almost impossible unless Hitler specifically designated him as the new Fuhrer. Even then, his tenure would be tenuous because of his strained relationship with the Wehrmacht. So he can't fundamentally change German society without starting a civil war. At least Himmler is a capable public speaker and could dispatch much of his party rivals with ease. Hess, Bormann and Goebbels would be easy to deal with, and Heydrich would have to be terminated. But Speer would be too valuable of an ally to purge, and Goering can't be purged without invoking the ire of the Wehrmacht, Luftwaffe and Kriegsmarine. Some disloyal generals can be purged here or there, but Himmler can't rock the boat too much or his regime would be in trouble. Himmler would probably make Speer his Deputy and would promote his own loyalists in the SS. But he would be forced to placate Goering and the Wehrmacht, so he can't introduce reforms that would be viewed as too radical. Himmler would seek to slowly de-industrialize Germany into an agricultural society and would also be even more ruthless with Generalplan Ost. Himmler would also try to be on peaceful terms with the West, but they no doubt would reject him. He would also try to destroy whatever is left of the Russians, and unlike Hitler, Himmler wouldn't have any qualms of using chemical warfare against what he viewed as lesser beings. But still, the future of the Third Reich wouldn't be that terrible under Himmler, largely thanks to the presence of the Wehrmacht.
Heydrich has long been viewed as a potential successor to Hitler, but it is largely impossible due to two big reasons. One is his strenuous relationship with the Wehrmacht, and they probably wouldn't be that keen of letting him be the new Fuhrer unless Hitler declared it as such. But the bigger reason is that Heydrich won't be able to publicly speak without revealing his embarrassing high pitched voice. Any leader that fails to be at least a decent speaker is doomed to fail. Sure, he can get rid of most of his rivals, but what would that really accomplish? Heydrich's best case scenario is that he purges Himmler and becomes the new Reichsfuhrer, while also having a puppet like Hess be the new Fuhrer. But again, we know how that story ends. The smart thing for Heydrich to do is to play nice and choose whoever he thinks will fulfill his ambitions. But he might also be too dangerous to keep alive, so his future would be uncertain.
But a Goebbels-led Germany would be the darkest scenario. While he isn't loony like Hess, Goebbels also wouldn't have qualms about rallying Germans to commit heinous levels of violence. His fanatical loyalty to Hitler means that there would be no possibility of an detente with the West. Goebbels' incredible public speaking and his ability to put people in a frenzy means that he could have all of his rivals lynched on a whim. The Wehrmacht and the SS wouldn't really have a choice but to accommodate the firebrand. Not even Goering would be safe from Goebbels' wrath. Goebbels' rule would mirror Mao's China, and we would see the sheer of extent of the Nazis' brainwashing of the German youth. The streets would filled with young people just as fanatical as him, and they'd likely commit bloody massacres based on the rhetoric of the small rabble-rouser. Goebbels being the new Fuhrer would be the textbook definition of a dystopia, and the rest of the world would just look on with horror.
Nazi Germany, had it won WWII, had multiple different pathways to the 21st century. But it all depends on who comes after Hitler.