If Gustav Adolf doesn't have any more children but still lives longer he could still arrange Kristina's marriage to her cousin Karl Gustav in any case. Not being queen she doesn't have the luxury of choosing to back out, she will no doubt do what her father wishes and what is expected of her...So perhaps Karl Gustav is still king in any case, just under different auspices. The question becomes...would they be co-rulers a la William and Mary or would he be Queen's consort only. Or would Kristina pull an Ulrika Eleonora...and abdicate in his favour.
I do think the 30 years war could well end earlier... most likely in a peace brokered in some small measure by France, diminishing the power to their rivals the Hapsburgs and securing some influence for the Swedish King among the protestant northern states. A loose alliance of protection is the best I can see. The French don't want the Swedes replacing the Hapsburgs and the Electors of Saxony and Brandenburg won't want to have replaced one master with another, they probably want more independence and influence for themselves. Sweden would probably gain predominant influence though among the smaller protestant states, particularly along the Baltic littoral...could they even gain all of Pommerania in this case then instead of splitting it with Brandenburg if things are settled say 10 or so years earlier.