Chara will perform a Grand Theft Me on Frisk.See this idea art by the author. Whether this would be consensual or forceful is unclear, as well as the consequences, but it woulg make for one heck of a Wham Episode.
Steven himself is a SAVE Point.Kinda obvious, but still a valid guess. Frisk mentions how easy it s to save around him, and they feel determined vicariously. Which makes sense. Steven is DETERMINED to save the wold as friendly as he can, he is DETERMINED to live up to the Crystal Gem ways, he is DETERMINED to make as many friends as possible. Given that he is one-half (Gem) magic, it makes sense that he is a living personification of DETERMINATION induced SAVE Points. In the same vein, this is why he remembers the LOADs. As a living SAVE Point, he'd remember the SAVEd data, as well as the variances that happened. Not to mention his experience with Garnet's future vision, which he might also be experiencing aftereffects of.