Scandinavia has a much lower tax base than the rest of Europe. It's climate means less agricultural output. It also lacks abdundant mineral resources that could compensate for that. In other words, it is poor. That is why the Vikings went to conquer other lands. Lastly, since Scandinavia was outside the Classical realm, it doesn't have the cultural institutions that would allow it to punch above its weight - it needs to inherit them from elsewhere. Scandinavia can't inherit that until strong states arise in Germany to pull it into the medieval Christian sphere.
Even if we assume all of Scandinavia becomes united and quickly develops strong institutions that counteracts its relative poverty to other European regions, it has a lot of competitors. So not only do you need Scandinavia united, you need to severely disrupt England, France, the Holy Roman Empire, Poland, and Novgorod/Muscovy for an extended period of time. Otherwise, one or more of those rival states will have access to greater resources and eventually supplant Scandinavia.
At best, Scandinavia could briefly dominate a Europe that has experienced a catastrophe that broke most of its rivals before it falls into its natural place as a second tier power. Essentially, this is what happened to Sweden who came into brief major power status during a series of calamties that wrecked most of the other major powers in the 17th century, but it quickly was knocked back down by the early 18th century.
Perhaps an earlier Kalmar Union combined with a more disastrous Mongol invasion of Europe in the 1240s.