What If:
Christina of Sweden doesn't abdicate?
And
What if her plot to conquer Naples had worked?
The second is a pet story of me, but I am a bit skeptical that she would have the necessary capacity for statecraft to do something useful in xvii century Naples, given that I recall her gofying a lot of estates yo courtiers, damaging the Swedish royal demesne and budget.
A way to achieve it would be slightly delaying peace between France and Spain.
Then there is the problem of her not wanting to marry for Sweden, why would she do it for Naples? (And at a time when she is in love with Cardinal Decio Azzolini)
France would press for a French prince as an husband, because Naples would obviously need French protection, at least initially. At the same time a French consort for Christina would not be liked by the locals and adoption seems a stretch and the best way to cause an ugly succession war.
It is not an easy TL to make realistic...