I stumbled across some interesting notes on the Congress of Vienna and the ambitions of several smaller states in Germany's north. Of these two plans are particularly noteworthy.
The first are the plans of Peter of Oldenburg, at that time only regent of the Duchy of Oldenburg, since his incapacitated cousin was still Duke. Peter apparently came to Vienna with the idea of not just defending the gains Oldenburg made in 1803 with the Reichsdeputationshauptschluss, but expand them further. He desired full control over the former Niederstift Münster, gaining full sovereignty over the co-dominions of Damme, Goldenstedt, Neuenkirchen, and Twistringen, as well as the former district of Meppen, which between 1803 and 1810 was ruled by the Duchy of Arenberg-Meppen. Of these co-dominions he would secure most of Goldensted, as wel las Damme and Neuenkirchen, but not Twistringen, despite it having been under the joint rule of Oldenburg and Münster previously.
Peter also desired East Frisia, a logical extension considering his aforementioned desires for Meppen and the assured acquisition of Bentick. During the Congress, he also brought up Arenberg-Meppen's exclave of Recklinghausen and the territories of the Free Imperial Cities of Bremen and Lübeck up. While he was apparently more than willing to concede these territories, seeing his claims as a mere bargaining chip, if one looks at the impact this maximalist series of claims would have had, it would have nearly tripled Oldenburg's 1810 population if they had been granted these territories.
EDIT: Here's a QBAM patch of the claimed territories. Please note that Jever, Knyphausen and Varel are shown as 1803 & 1815 gains for the sake of simplicity, since at times they were controlled by Oldenburg pre-1803. I also marked the Herrlichkeit Dinklage as an 1815 gain, though it was largely treated as part of the Oldenburger Münsterland IOTL and thus arguably was added to Oldenburg in 1803. Also Twistringen and Grolland are marked as "Desired Gains", even though both were part of Oldenburg/ruled in part by Oldenburg before 1803. Plus Grolland would be too small to be shown individually anyway.
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But this is nothing compared to what Ernst Friedrich Herbert zu Münster, Hannover's delegate to the Congress, desired. Starting during his exile in the UK in 1809 and until 1813, he dreamed of not only restoring Braunschweig-Lüneburg, he dreamed of elevating it to a kingdom that could stand on its own, even if the personal union with Britain were to end.
He preferred to call this enlarged, empowered Braunschweig-Lüneburg Austrasien, and if you know your Frankish history, you're familiar with the name and understand some of the implications. He wanted Hannover to rule Germany "between the Elbe and the Scheldt", meaning that Hannover would have gained significant chunks of modern North Rhine-Westphalia and Saxony-Anhalt, as well as the Netherlands and Belgium. Zu Münster, along with the Prussian commander August Neidhardt von Gneisenau (who lost influence & favor in 1815 due to his radicalism), were hoping that such a strong North German state could prove a suitable counter towards future French aggression. Prussia meanwhile was to be limited to an area between the Elbe and the Vistula, with Russia gaining East Prussia and large chunks of Poland.
However after 1813, when it became that Prussia was bouncing back, the British urged zu Münster to back off and present more moderate demands.
He ultimately would outline the need to gain the former Bishopric of Hildesheim (which Hannover did end up getting), the Principality of Minden and the County of Ravensberg (both Prussian, neither of them gained), the Hessen-controlled County of Schaumburg (also not gained, but instead Hannover would get some bits of the Eichsfeld and the County of Hohnstein from Prussia), as well as East Frisia and Meppen, and a land connection to the Grafschaft Bentheim, which Hannover was also almost certain to permanently gain. To facilitate the gaining of East Frisia and sections of Greater Westphalia to connect Osnabrück to Bentheim, he agreed to the series of horse trading deals that, for Hannover, involved the loss of Lauenburg to Denmark.