It is quite possible to keep Sweden a grand power for quite some time.
We start by the Great Northern War. By 1708, things were looking bleak for Sweden's enemies.
Denmark was out, and was renting its army out to the naval powers and no threat.
August had been forced off the throne and a Swede-friendly King had been installed in Poland-Lithuania.
Russia had not won a single field battle against Sweden and had only managed to make inroads into Ingria - all attempts at Estonia and Livonia had been defeated and Russian armies sent to aid August in Poland-Lithuania had been more or less eradicated.
If we change the outcome of Lesnaya and Poltava to Swedish victories - which is not that hard, btw. The Swedish army had suffered during the winter 1708-09, but were still very much a competent force.
A Russian defeat in these two battles, especially if you add a successful Swedish pursuit might force Peter to sign a peace to placate his own conservative opposition.
Sweden retaining its territory, annexing Polish Livonia and the Duchy of Courland (from Poland-Lithuania) and Archangelsk, Far Karelia and Kola from Russia, probably holding Pskov or some other city as a guarantee of Russian war reparations.
After the death of Peter the Great (which will probably not be considered great in this timeline, after defeats against the Swedes and Ottomans) Russia OTL entered a minor times of trouble again, and it was only the institutions built by Peter that allowed Russia to remain a modern nation. If he has been forced to dismantle them, Russia could revert back to a semi-Asiatic power, permanently behind western powers in technology and organisation.
Sweden looking strong and Russia being weaker could have Russia look south, to fight the Crimean Khanate and the Ottomans, probably with less success than OTL.
Russia's ascension as a European grand power could be seriously delayed or even prevented completely by this.
Poland-Lithuania is in a state of decline and will probably use up the last of its power fighting the Ottomans and the Russians rather than the Swedes. It will not be a threat to Sweden.
Denmark will be in a permanently weaker position compared to Sweden, and will lack a strong ally on the other side of Sweden. It is possible that some kind of revanchist war will happen anyway, and Sweden emerging victorious, perhaps taking Norway.
German unification is not unavoidable. The extra weight of Sweden attempting to take more of Pommerania while still being strong could be enough to crush Prussia during the 7 years war. Germany will then be united more loosely, and from Austria. It is possible some northern German states will remain out (perhaps Hannover if under British rule) and Sweden keeps its German posessions.