I think you need a POD which boosts prosperity at home- IIRC much of Scandinavia was dirt-poor up to the 20th Century as opposed to the relatively affluent, highly urbanised Dutch population which allowed the Netherlands to punch above it's weight for a century or two, as it were.
Another option is to cheat a bit- my Anglo-Saxon TL from 2005 (where the Norman Conquest failed) saw the development of a line of Anglo-Saxon-Scandinavian kings which drew England, Scotland and Scandinavia into a United Kingdom of the North. But that's cheating since it's like saying Scotland was a Great Power when it was only one as a constituent part of Great Britain.
Norman conquest, played correctly, might help. I think Harold Hardraada might be a good possibility. Something like this might work:
Harold hardraada's campaigns against the danes are a little more successful, and Denmark is reduced to basically a vassal state. This takes some time, however, and Harald cannot immediatly move on England when Cdward the Confessor dies. This isn't necessary, but it ensures that Harold Godwinson is more commited to standing in the south of england. Then, it comes down to the timing of his strike. If he lands before the Normans, then Harold makes his forced march with the huscarls as in OTL. However, at the stamford Bridge analogue, have Harald bring up his entire forces, ready for battle. Despite being divided in two by a river, not being armed for battle, and probably missing part of their army (sources vary on this one), the norwegians fared rather well against the Saxons. Have the vikings a little more prepared, and Harold Godwinson and his army could very well lie dead by stamford Bridge. Harald then marches south to oppose William when he lands. This could be more of a tossup in terms of the outcome, but I can easily see a norwegian victory here. The numbers would be roughly equal, but Harald would be commanding a battle hardened and victorious army. He himself was an accomplished commander, and probably more then a match for William.
If, on the other hand, he lands after or at the same time as the Normans, things are easier for harald. He probably defeats edwin and Morcar at Fulford as OTL, and Tostig Godwinson joins him. But this event gives him time to consolidate his control over Northern and Central england, and to move against the victor. This puts harold in a tougher position all around, and let;s assume that the Normans crush the Saxons roughly as in OTL. Now, Harald is fairly well established in England. His Norwegians are culturally, linguistically, and ethnically closer to the saxons then the Normans (granted, the normans were descendents of vikings, but influenced by french culture; but at the time the Norwegians would have been more similar); and his alliance with Tostig might be able to lend him some credibility, since Tostig is the Only saxon relative of the king with any real power. Hardraada can chose when to fight the normans, and probably will win.
So where does this leave us? England stays somewhat Saxon, but is a part of the Norwegian Kingdom (probably somewhat autonomus, but still a part). Norway has defeated Denmark, probably securing the entrance to the Baltic and vassalizing Sweyn/his successor in the process (the HRE will not be pleased). Norway is overall ascendent in Scandinavia and the North sea. Give it a bit longer for this kingdom to slowly become permanent, probably through conquest of or vassalization of Sweden and Denmark. Norway is now rather well set up to become a great power.