Option 1: When 1st-year Harry arrives at Hogwarts, this has always been Snape's subject of teaching.
Option 2: Harry starts a new year at Hogwarts and it's announced that Snape is now teaching that subject instead of Potions.
- Must be in-character.
- Keep Snape's skills/experience intact. Whichever subject he's teaching must be something that makes sense for the skills and knowledge he has - such as Herbology. You can adjust them if necessary, but don't give him entirely new skills or remove canon ones to justify which class he teaches. Headcanons and interpretations of his canon skills or potential are allowed.
- Harry must take the class (such as, if it's an elective like Care of Magical Creatures than Harry should at least be in 3rd year).
- Minimum of a few chapters. However, this challenge does not need to be the focus of the story itself (it can be - but that's optional) just a prominent aspect. If you want to combine this with other challenges or plots, go for it.
- Snape is still Head of Slytherin.
- Harry should attend the class often throughout the story - not just once or twice shown and that's it.
It's up to you if Snape chose to teach this subject and enjoys it, or not. And how Harry performs in this class, based on his own skills and/or Snape's teaching methods - and does he therefore do better/worst in Potions with someone else teaching it? What about Neville? Snape can still brew potions in his spare time and have his own storage cupboard, as well as give Harry Potions advice. There's a lot of potential for different interpretations and character interactions here so have fun!