As per my understanding. Since Xel'naga was responsible for creation of both species and as the primal Queen of Blades, with her psionic abilities, she might be unrivaled by any being. Sarah did have power beyond Xel'naga at the end. As from the beginning, she was able to spin zerg DNA strands/sequences into her own creations. Also had very swift regeneration during the time when her wing was severed, after Xel'naga form by consuming the essence of Ouros which Biologically refers to being near immortal. The only reason Ouros faded from existence was by will, transferring the essence to Kerrigan.
Overmind planned and considered Sarah as her daughter and the cycle would be broken only by her. Leaving her to create a future free of the despised Xel'Naga. This was eventually accomplished. Upon slaying Amon, she could've been reborn at will any moment. Just as Ouros, Sarah had metaphysical reproduction leading to new iterations of the species that can reappear in the Cosmos on a cyclical basis in new forms.
Also Kerrigan's vowed, even when she believed Jim was dead. That if Jim was still alive, nothing in this universe will stop her from finding him. Later on, from her rematerialisation, not knowing the outcome of the end battle. She could've still sensed him out there. Since she could take any form to visit him, my guess from how it all began, to start again.
At the end credits, Kerrigan's fate remained a mystery. Even Jim wasn't seen anywhere. Though you can hear about a new unknown planet being created. It probably is where you can find Adam and Eve, which is the creation of the next generation of Xel'Naga and beginning the cycle anew.
I guess Blizzard could not predict the future of the franchise, resting it all in the safe zone and leaving us to decide on with two end topics being the A Queen's Legacy, where Niadra remained loyal to Kerrigan's final directive to destroy the protoss, also even taking the brood into battle against the Dominion of Terran. And finally Ghosts of the Past, about Sarah's appearance as during when she was the Confederate Ghost. It hints at the Orna III mission as well, when she claimed to be a Ghost who had captured Raynor, the criminal. But it's also where Raynor and Kerrigan and the two gave into their emotions. The scene was described as Lovers under starlight in Sketches. Should be most fitting to the image on the new planet.
Like in graphics? Were they made in a higher Pixel Density than earlier? Not to mention the curves of Queen of Blades are far better now.
