yeah, one of the last realistic tipping points for bringing Denmark back to being considered a second tier power would be solving the Napoleonic wars in a different way ... One way or another Britain have to be stopped making an enemy out of Denmark, either by not considering attacking them in the first place, or by Russia pulling up aggessively telling them not to and point them towards what they did doing the first league of armed neutrality (doing American Revolution), and tell them that they wouldn't be scared of pulling the same tricks again, with Britain buying it as they would be aware that they wouldn't be able to fight everyone at the same time.
Long story short, stop Britain from forcing Denmark to ally France (or optionally, stop France from losing, prehaps by killing of Napoleon before he gets too greedy) and you're half-way there ... specially if you somehow manage to align Denmark and Russia against Sweden using the fact that the rest is to busy to bother, to carve up Sweden (Giving Russia Finland and Swedish baltics as otl, and Scania, Halland, Blekinge, Bohuslän, Jämtland and prehaps even Götland to Denmark)