It is possible for several German states to have several decades of successful foreign and domestic policies to become able to challenge Austria.
Maybe Saxony gains Silesia in the 18 century, enabling a more permanent personal union to become a European Power.
Bavaria managed to become German Emperor for a few years in the 18. century. If they might get more foreign support, they might maintain that title? Also if they somehow enlarge their territory (for example gaining Ansbach-Bayreuth) they could have better chances.
Also an earlier unification of the Palatinate with Bavaria would massively increase their power.
I don't really see a path for Hannover.
Württemberg could maybe get a more important role, but I don't see them dominating Germany.
I disagree.
Hannover was in a personal union with the United Kingdom. This gave them additional resources they could've drawn from if they had chosen to make German unification their main mission. The only serious opposition they would've faced was, on the one hand Prussia, and on the other the Hapsburg monarchy, With the Royal Navy to defend them from possible Danish or Swedish attack, Hannover could've begun bringing the surrounding small states into union, then challenged Prussia for supremacy in northern Germany, with the possible result that a North German Federation might've been created incorporating Hannover, those parts of Prussia not directly annexed, and the other small states. Then its only a matter of either going against Austria or France as a way to bring the southern German states into the union.
Bavaria only managed to attain the title of Holy Roman Emperor because they had seized Bohemia, but if they did that again, and also acquired Ansbach-Bayreuth and the Palatinate, they could win and hold that title again and translate it into a German Empire.
Personally, I don't see how Saxony or Wurttemberg could unify Germany. In Saxony's case, their main concern would be in linking the electoral core territory with the PLC and hence enlarge their dominion. Its doubtful the Poles would welcome a united Germany on their doorstep even if the ruling family came from Saxony, although a personal union of the German Empire and the PLC would be the largest empire in Europe and likely frighten Austria (or what might remain of it), Russia, and France.
Wurttemberg would have to enlarge its territory enough to become a serious contender against both Prussia and Austria and its likely Bavaria, Baden, Austria and potentially even France would work to prevent that from happening.