Seems difficult. Sweden had has almost a hundred years of internal political turmoil, it's deeply corrupt, with a starving population and feeble economy. The professional Carolean army is gone and what's left is outdated.
We're gonna need a PoD early after the great northern war to reverse this. A more competent king leading to a earlier gustavian restoration of power could be a start, it removes a lot of ways foreign influence would affect Sweden atleast.
Any gains Sweden make on Prussian expense are probably moot btw unless Napoleon wins, but Sweden on old Nappe's side doesn't necessarily mean this.
As for this potential Empire... well Norway will still be highly autonomous, it might actually gain independence sooner since Sweden will have enough trouble keeping down German and maybe even Finnish nationalists.
I do think Sweden could have remained a european power post-great northern war, but I also think the Napoleonic wars are too late a PoD.
I agree, and I actually think Sweden needs to end the Great Northern War much sooner with treaties with Russia and Poland-Lithuania shortly after the Battle of Narva. Furthermore, I think Sweden needs to further prevent Russia from ever gaining that window on the Baltic that Peter the Great went to war with Sweden over. Sweden may never have had the money or resources at the time to build their own version of St. Petersburg (though they did have Nyen), but they just need to keep Russia out of the Baltic in order to maintain their great power status.
If Sweden can do that, if Charles can avoid losing his army later on and if they can actually build the numbers back up, Sweden could get a chance to weigh in on the War of the Spanish Succession when it would best suit them. I anticipate that at the point France is at it's lowest ebb, Sweden would join the Grand Alliance and secure their own gains and territories they gain from Denmark-Norway, Poland-Lithuania, and Russia, along with also securing the lands that were coveted by Hanover and Prussia. It would be a backstab of France, since the French got them a lot of lost territory back in a previous war, but Sweden needs time to fully rebuild their forces, especially since Russia would be looking for another crack at the Baltic. The pragmatic decision is to go against the French here.
So France here likely suffers a clearer defeat, but with Prussia involved in the treaty, I don't think there's any way they would allow Sweden to gain any more of Pomerania than they already have. Sweden and Prussia had warred over Pomerania before, seeing Sweden hand over parts of Swedish Pomerania. Sweden probably takes the rest of Courland, creating another border between themselves and Prussia, and here's where things become interesting.
Sweden and Prussia become THE rivalry in the Baltic and in any future wars involving Great Britain, France, and the HRE, I see Sweden and Prussia usually being opposed to each other, vying for influence over the other states in the Baltic, while both also worrying about a resurgent, ambitious Russia which would probably make further gains against the Ottomans if Sweden can continue to keep them away from the Baltic. Nyen would become their bulwark against Russia, as well as a network of defenses from Nyen to Lake Ladoga.
However, they would need to be careful about which battles they choose to fight and which alliances they need to make. Sweden is at a manpower disadvantage to both Prussia and Russia and will remain so for the whole 18th Century, even with their holdings in Finland, Ingria, Estonia, Latvia, Livonia, Courland, Swedish Pomerania, and Swedish Bremen-Verden. So I do see a Swedish-Habsburg alliance for sure, France likely making alliances with Prussia and Russia, with Great Britain probably throwing in with the Habsburgs as well, despite Sweden holding territory that the Hanoverian rulers of Great Britain wants.
But this is just the most pragmatic timeline, one forged by pragmatic, patient, defensive decisions. The young Charles XII wasn't one for pragmatism, but if he can be reined just a little by his advisors and develop into a more patient, wiser king, perhaps Sweden could have remained a Great Power, possibly even one with colonial ambitions if they can make their way to the North Sea.