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.
Don’t know about ice cream, but adding mixed frozen cauliflower to chocolate milkshake is invisible to the taste for me (and I am a picky eater) and I find it much more filling. Some spinach work too but you can add too much before noticing it by taste/smell Even at a third, cauliflower is still invisible to me in comparison.