Can another sweetener be substituted for the granulated sugar?

Hi Eliza! All the granitas I've made have used granulated sugar, so it's hard for me to say for sure how something like agave would act in this recipe.

My hunch: Since agave is a liquid sweetener, it actually might work against the classic flaky texture of a granita, yielding something creamier than intended. (Liquid sweeteners—like corn syrup, agave, honey, etc—lower crystallization in frozen desserts.) More on that in this great article by Max Falkowitz on Serious Eats: https://sweets.seriouseats.com/2013/07/the-deal-with-corn-syrup-makes-better-sorbet-why-use-invert-sugars.html