The 'population' problem is something that's hard to avoid, yet I believe it all depends what kind of great power you want Sweden to become. Snatching up Finland would give it a nice buffer towards Russia, though you'll have to keep Russia weak to avoid Finland being retaken.
Russia really is the elephant in the room, as it did not wish a powerful Sweden. Maybe an 'Ottoman' Russia in constant, slow decline, with a powerful, possibly westernized Ottoman Empire to grab at the heels of the weakening giant? The Ukrainians go for Ottoman support for their independence rather than Russian, Ottoman Empire successfully keeps their Russian enemies at bay. A Swedish-Ottoman (and maybe -Prussian or -Austrian?) Alliance aimed at keeping Russia weak would go a long way to avoid that kind of competition.
Or just have the Russians never unite, Moscow never gets a chance a greatness and its all small dukedoms and unexplored 'wilderness'.
With Russia somehow taken care of, the Swedes will dominate Scandinavia. Norway and Finland are obvious targets, Denmark isn't really important for anything more than control over the Oresund strait. And that kind of control won't matter too much if the Swedes can take Norway and use Oslo/Christiania as a port towards the rest of the western world. Good Norwegian timber will give a nice export boost.
With a population of some 10 million and lots of timber for ships, good iron for cannon Sweden could potentially match the Dutch trading success. An alliance with Great Britain or France, or an armed neutrality to avoid colonial conflict between it and the great powers? Being the third rate power, allowed to take what the Great Powers don't want or can't decide how to share, wouldn't be all bad.
That is an economic powerhouse right there. It probably won't win a lot of great military victories or become a hugely influential mover and shaker of the world. A comfy economic power with a nice military to back it up isn't shabby either.