Spitballing an AU idea (that needs more angst and seriousness than I’m good for)
Severus Snape time-travel AU, but from Tobias’ external perspective (probably a less abusive Tobias than whatever it was Pottermore implied. Messy and awful and not liking anything much and estranged from his son for nearly a decade despite sharing a roof, but also just a human who'd had the world yanked out from under him and hasn’t yet stopped falling). So, how would a Severus Snape who’d just died thinking he failed (failed Lily, Dumbledore too, perhaps) after he’d spent a suffering year as the Headmaster at Hogwarts react to having another chance (albeit from after SWM) and what would that look like from the outside?
A mystery story: Who is this Severus? What has happened to his son, aged sixteen, home for the summer, only yesterday so full of rage (still so hurt over Eileen’s death last winter) and somehow no longer friends with that Evans girl? Why did Severus save his father’s life (and how had he known where to be?) when just the other day the boy had glared like he wanted to put Tobias in the grave himself? Can you build a father-son relationship on a foundation of sand and ruin when the son has suddenly become a stranger with some personal mission to stop a war and save the very people that wouldn’t’ve spit on him if he were on fire (that boy is acting like a soon-to-be saint grimly rushing towards martyrdom!)?
Years ago Tobias hadn’t wanted to believe in magic, when it was undeniably real, he’d resented magic (he still didn’t like magic). But Severus is acting like it’s his job to save the magical world and maybe there's a chance that he might need his muggle father, maybe it isn’t too late for Tobias
(I like the image of Severus seeing Tobias’ approval in the Mirror of Erised. I like the idea of a very human Tobias who was utterly blindsided by magic being a thing and in over his head and still, possibly, capable of being better. But I’m not terribly prone to making angsty things or taking stories seriously for very long at all, so if it sparks any interest feel free to grab and go running)