I guess the two things I would like to see most in an anal addon would be more vigorous thrusting animations than the current vaginal sex positions and more intense facial expressions. In theory, Jak could just modify almost any current position by moving the hips slightly to have the penis or strappon go into the anus instead of the vagina to make an addon pack. But when I think of awesome anal sex, I think of a couple that's really going at it. They're pounding, moaning and screaming, and slapping against each other with the intensity (hopefully with lots of lube). (love the sound of intense, lubed up sex)
Ideally the voices might even become more intense and louder, but I would understand if that might prove difficult to implement with the current setup. Currently the voices don't change when you change position. Though they do change when you hit different benchmarks in their pleasure meter. So I don't know how much of a pain it'd be.
We can change the speed of all of the current sex positions, but they tend to stay toward the slower, gentler end of the spectrum. The animations themselves seem geared toward this speed in my opinion. They're sensual, intimate, and fairly gentle. I like them a lot, make no mistake about it. But I think it would add a lot of spice and variety if an anal addon pack brought an avenue to ratchet up the intensity a bit. I would be much more excited for that than just the same kind of animations aimed at a different hole.