As a low-effort PoD, perhaps Victoria is born a man, keeping Hanover and the UK in personal union. A viceroy is appointed that is as popular as the OTL Ernest Augustus, who during the 1848 revolution was able to defuse the revolutionaries simply by threatening to abdicate, so that British presence in Hanover is able to be internally stable. That way, whatever pan-German sentiment that springs up (perhaps harnessed as a “third force” of the medium German states such as Hanover, Bavaria, Württemberg and Saxony, or a strengthening of the OTL Steuerverein) have the unofficial protection of the British empire, ward Prussia off and slowly head for a slow confederalization or unification of the non-Prussian bits of Germany.
I agree with this idea, in particular because we have evidence that in Otl the Habsburg court repeatedly tried to involve Great Britain ( under the regency of George III and the reigns of George IV and William IV ) to seek greater influence in the confederation, so as to make Hanover is a solid opponent of Prussia, as well as an aspiring leader of the "third Germany" who potentially compromises with Bavaria ( another claimant to the title, in OTL it is the main promoter, together with Wurttemberg and Saxony, of greater collaboration between the smaller German states ) to better coordinate, both with the two failed attempts to make Hanover the admiralty of the north of GC, or trying to pass federal laws that would strengthen the middle states ( Vienna was not interested in expansion, unlike Berlin, rather it was content to maintain the status Quo, where it was already recognized by custom as the leader of the German states ) finally in a hypothetical ATL 1848, it cannot be excluded that a skilled sovereign could exploit popular feelings to gain influence and territories for himself ( because honestly speaking the survival of the multitude of minor states, the so-called Kleinstatatei "after 1848 favored in the long run Prussia, because they could not represent a serious threat to its aspirations, leaving only Hanover to face it against it, on the northern front of the war, thus being able to concentrate the bulk of its army against the Austrians and Saxons ) so to recap if Hanover had in any way having obtained control over the various Lippe, Waldeck, Anahlt ( 1 ) and pushing for an enlargement of Saxony into Thuringia or possibly unifying said territories into a single state, it could have become a good means of strengthening the third Germany and at the same time the possibilities that in the event of a future war between Austria and Prussia, the latter does not have much chance of victory
1 ) even just gaining control over Brunswick in some fashion around the 1820s might slightly change things, given that the inhabitants of the duchy seemed truly split between wanting to remain under the control of Hanover (and GB) or supporting the political autonomy of their young Duke Charles II ( who was much loved at the time, but who was later driven out with the uprisings of 1830/31 )
P. S
also honestly there would be infinite possibilities to have a Germany not unified by Austria or Prussia, it may seem strange but there are pods that we can use even from the 30 years war / Schmalkand (there is one thought of by the Viennese court of the period which would have seen HRE transformed into a more centralized state but in a federal sense with the simplification of many secular territories and their subsequent union into larger confessional entities ( the Habsburgs wanted to definitively resolve the problem of the Augsburg agreements, creating individual potentates for a specific Protestant confession, therefore a Lutheran kingdom where all its adherents should have converged, a duchy for the Calvinists, a buffer state for the Catholics between the Habsburgs ( including the Spanish branch ) and France and finally a slight imperial expansion in Swabia
