Brew father and BeerSmith have free/trial versions. You could take a look them for an idea of what's already out there. They have pretty different interfaces but perform a lot of the same calculations.
Both of them, however, are pretty beer-centric (if you couldn't guess from their names...). If you wanted to differentiate yourself a bit, you could focus on integrating mead, cider, and wine making. Maybe I'm just doing something wrong, but BrewFather, at least, doesn't seem to handle oeno-beer hybrids, graf, braggots, or pyments/melomels very intuitively.