Unsweetened or fruit sweetened ice cream - Replacement for sugar in ice cream?

From a bulk perspective you don't have to replace sugar with anything, you certainly don't need want to add starch to compensate as that will not do favors for your consistency and mouth feel. Sugar helps the consistency of ice cream by reducing ice crystal formation, protein does not, and starch does a bit but makes your ice cream, well, starchy rather than creamy if you add much. Vanilla extract doesn't inhibit ice crystal formation much as it's not enough alcohol unless you add way too much, straight alcohol like vodka is a better choice.

For me the solution isn't bulk, but to make up for the lack of sugar using a stabilizer like guar gum and/or carageenan. Locust bean gum is good for presenting ice crystal formation as well.

Regarding sweetness, fruit isn't a good source of sweetness because of concentration. Most of fruit is water, and to add enough sweetness and fruit flavor you have to add a lot, adding a lot of starch, cellulose and water to your ice cream which isn't good for consistency. When I add fruit for flavor I cook it down into a jam to reduce its water content, this works well for berries, but wouldn't work for starchy fruits like bananas, you might try banana chips instead. I think what without some sort of sweetener it's going to end up pretty bland though.