I like adding things to my icecream, usually peanut butter and frozen fruit. Got to thinking that if I added oats I could actually increase the volume without impacting the flavour all that much (I like oats). I could probably use floured starches or something like that.

Are there other things you "fill"? I think juice + water is the most familiar example. What about something like adding 20% dehydrated milk to fresh milk? Substituting some butter for oil?

Sometimes I find when I'm making my own stuff it ends up being more expensive than buying the packaged variety from the store, but maybe fillers are a way to balance that out.