One cool feature would be how to handle substitutions. For example, if I am trying to make a pie crust. And I don't have butter, it would be good to be able to cross out butter, and see crisco or vegetable oil added to the list. Or by adding Crisco, see that the butter is removed from the menu. Because a Pie crust recipe without shortening does not a pie crust make.

However general baking is more difficult than cooking, since the ratios are so much more important. But it also means the app would have so much more value added in this context.