Its going to be interesting religiously as while their got a couple of centuries tech advance that's not totally overwhelming but they are an isolated heretic faction now. Might even end up siding with the Russians - or possibly was Lithuania still pagan - to face up to a possible crusade.
They do have the advantage of much better guns and ships and the printing press as well as a much wider knowledge of the world beyond Europe than the people now around them.
What are we taking about for Sweden? I think it had Finland by this stage and possibly some parts of the northern Baltic states but did it have all of modern Sweden? I know a couple of provinces in the SE were still under Danish control around this time so they may still be from 1400 and think of themselves as at least partly Danish? [~20 years ago I played the Danes in a PBM game set around the 30YWs and think the two provinces were still Danish then in 1618.]
One issue is would there be any prominent people in Sweden who for whatever reason, private feud with Gustavus, religion, personal advantage or whatever who might try switching to any down-timers bringing technology/knowledge with them?
From what I read Gustavus was really shaping his Army during the war with Poland 1621-29 making it the proficient instrument of War later seen during the TYW in German lands.
Sweden and Denmark-Norway 1617:
Sweden is both the yellow and the pale areas though I've seen other maps where Estland is no longer Swedish than to the north of Lake Peipus which is just east of Dorpat.
Christian 4 had been able to pull off a small victory in the Kalmar War 1611-13 mainly because of his modern Navy. Gustavus had to rely on his countrymen for troops as Denmark barred the route to German recruitment centers!
Kalmar Union 1400:
Regarding modern inventions - guns are mentioned in Denmark 1427 though real Navy ship guns doesn't appear till 1493. There may of course have been some used during that time span and sligthly earlier however Artillerymen are mentioned 1490. All pieces were small guns firing crossbow dart and lead bullets. Real guns appear 1493 with bronzeguns being cast in Copenhagen. This is the time of European warships getting real guns.
Swedish Navy was rebuilding as it had fallen in decay since Eric 14. 1620 would be the year Gustav 2 initiated the building of a new Navy which by 1625 had 25 new ships - so Swedish Naval force round 1620 still consisting of older smaller ships with the new Navy not to make a show till 1625. The first campain of the War with Poland-Lithuania is in Livonia/Livland south of Estonia/Estland.
The thing is that Gustav's prime enemy Sigismund of Poland isn't around; the troubles with Poland-Lithuania began as Gustavs grandfather Gustav Vasa had three sons Eric 14, Johan/John 3 and Karl 9 who were at each other throats. All three would become King of Sweden. Eric then Johan then Johans son Sigismund who would become King of Poland-Lithuania 1587 and of Sweden 1592-99 to be succeede by his uncle Karl 9.
Really the Swedish - Polish/Lithuanian wars was over the succession to the Swedish Throne. The war kept going to 1629 when peace between the parties were made by French mediation leaving Gustav to turn to TYW to succeed as the Prince of the Protestants now Christian 4. of Denmark-Norway had been defeated and succeed in withdrawing from TYW by making overtures to Gustav of a joint campain but withdrawing from that too 1629 leaving the battlefield to Gustav.
OK- with two major obstacles removed Gustav have a free hand in the Baltic area. He have to rebuild the Navy before he is really able to persue such action even if he still have some ships available though OTL he remains in Livonia till 1626 and then moves across the Baltic to the mouth of the Vistula and runs the second part of his Polish campain.
He may still consider going for Scania, Blekinga and Halland instead of going across the Baltic to Estonia/Livonia. Regarding his Protestant belief and having lost the prospect of becoming the Prince of the Protestants in Germany I'm not really sure his is going that way.
Based on his knowledge of the time elapsed from 1400 to 1620 he may go for making the Kalmar Union a Swedish run business; he won't come at odds with the non-existant Royal and Dutch Navies of his time and their interests in the Baltic area and may place himself astride trade in Northen Europe. With a very narrow land border he may then go east to secure the outlets of Russian and Polish-Lithuanian lands of continental trade for himself.
Though he won't sharpen his sword as OTL he will still have the most proficient and additionally a national Army and Navy.
Then back to the religious issue; Gustav may have some trouble in getting his Danish-Norwegian subjects into his boat due to religious differences. But then Denmark was very late in adopting tithe and Norway christened only 1020 and all Scandinavian countries were only really party to the Christian Church by 1300. He could make some statement to the Nobility by letting it have major church lands as a bribe.
Gustav will have another kind of problem as he wanted to marry a Protestant and thus went to Brandenburg spring 1620 to propose and arrange for marriage with Maria Eleonora of Brandenburg which her mother accepted.
Outlining a few things the timing of the ISOT is quite important as a lot depend on weather as in the real world and mostly so when moving around chips in the past.
