Not focusing on 'how' Sweden was able to hold its empire, but what if it still was an empire in the 18th century.
Or maybe the Swedes decide to prop up Poland-Lithuania as a counterbalance to Russia, if we go by a Great Northern War PoD.Perhaps Sweden gets its own little slice of Poland in the partitions?
A Swedish Gdansk and not much else would be an interesting sight.
Not focusing on 'how' Sweden was able to hold its empire, but what if it still was an empire in the 18th century.
One of the biggest problems is that Sweden was always punching above its weight, with foreign mercenaries taking an even greater role in their army than in other countries. Sweden simply doesn't have the population to compete with countries to their south, especially Russia, over the long term. How long Charles XII and his successors can keep it up is uncertain, but Russian reforms to their military were already under way, and Swedish qualitative superiority can only count for so much.
For Sweden to remain a great power they must prevent the rise of not only a east Slavic power but also a German one as a suitably large east Slavic or German state with make them irrelevant