Alternate Shadow and Bone idea: The sun summoner turns out to be an old lady from a quiet village, who sensing she didn't have long left to live, hired a skiff and decided to brave the journey through the Fold in hopes of seeing her estranged extended family on the other side. When the volcra attacked, her fear set her power free, which until then she was ignorant of because it only came out as burning things when she got frustrated with cooking. Because of lack of education in commoners, she too considered Grisha abnormal witches until she was taken to the palace by the extremely disappointed Darkling. She sees the Grisha children, orphaned, hunted and abused for existing anywhere else, the Darkling running around to keep the country from collapsing while the King sits on his ass and decides to do something worthy since she has practically waned away her whole life anyway.
Her training progresses slowly because of her age and not because "wahh, I can't live without Mal for a month". Also because she and Baghra have the biggest old lady beef and bicker all the time. She's a better mother figure to Aleksander than Baghra. Kills the stag with zero hesitation because she had to butcher animals and store the meat for long winters. Has beef with the Apparat as well and tells the brainwashed children that he gathered in a cult to wise up and go get a life. Doesn't blame Genya for doing what she had to to survive and doesn't guilt trip her to take her side. She understands because her parents married her off when she was young, fortunately her bastard of a husband died soon after. Becomes friends with Ivan because paranoid Darkling assigned him to keep her heartbeat and blood pressure in check, so the old sun summoner won't randomly die. Is not impressed with Nikolai when he comes back to Ravka at all. In her opinion, everybody has done more for Ravka than him and he doesn't get to swoop in, decide he wants to be King and take all the credit.
Ending: Either gives her powers up because she's old and tired of life or leaves after things settle down, continues living for centuries as a healthy old woman because of her powers and becomes a local legend like Baba Yaga. She and Aleksander keep in touch, he comes over for tea, пирог (pie) and advice.